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Heat Exchanger (G-TL)

Heat exchanger for systems with gas and thermal liquid flows

  • Heat Exchanger (G-TL) block

Libraries:
Simscape / Fluids / Heat Exchangers / Thermal Liquid - Gas

Description

The Heat Exchanger (G-TL) block models the complementary cooling and heating of fluids held briefly in thermal contact across a thin conductive wall. The wall can store heat in its bounds, adding to the heat transfer a slight transient delay that scales in proportion to its thermal mass. The fluids are single phase with pure gas on one side, and pure liquid on the other. Neither fluid can switch phase and so, as latent heat is never released, the exchange is strictly one of sensible heat.

Sensible heat exchangers abound in machinery. The fuel heaters that in some jets keep ice from precipitating in fuel lines and from choking fuel strainers work by blasting bleed air still hot from the compressor over the fuel lines. The oil coolers that in some motorcycles keep lubricating oil from overheating work likewise by rushing ram air at ambient temperature over the oil lines. The bleed and ram air are gas flows and the fuel and oil are thermal liquid flows.

Heat Transfer Model

The block heat transfer model derives from the Effectiveness-NTU method. Heat transfer in the steady state then proceeds at a fraction of the ideal rate which the flows, if kept each at its inlet temperature, and if cleared of every thermal resistance in between, could in theory support:

QAct=ϵ QMax,

where QAct the actual heat transfer rate, QMax is the ideal heat transfer rate, and ε is the fraction of the ideal rate actually observed in a real heat exchanger encumbered with losses. The fraction is the heat exchanger effectiveness, and it is a function of the number of transfer units, or NTU, a measure of the ease with which heat moves between flows, relative to the ease with which the flows absorb that heat:

NTU=1RCMin,

where the fraction is the overall thermal conductance between the flows and CMin is the smallest of the heat capacity rates from among the flows that belonging to the flow least capable of absorbing heat. The heat capacity rate of a flow depends on the specific heat of the fluid (cp) and on its mass flow rate through the exchanger (m˙):

C=cpm˙.

The effectiveness depends also on the relative disposition of the flows, the number of passes between them, and the mixing condition for each. This dependence reflects in the effectiveness expression used, with different flow arrangements corresponding to different expressions. For a list of the effectiveness expressions, see the E-NTU Heat Transfer block.

Flow Arrangement

Use the Flow arrangement block parameter to set how the flows meet in the heat exchanger. The flows can run parallel to each other, counter to each other, or across each other. They can also run in a pressurized shell, one through tubes enclosed in the shell, the other around those same tubes. The figure shows an example. The tube flow can make one pass through the shell flow (shown right) or, for greater exchanger effectiveness, multiple passes (left).

Other flow arrangements are possible through a generic parameterization based on tabulated effectiveness data and requiring little detail about the heat exchanger. Flow arrangement, mixing condition, and number of shell or tube passes, if relevant to the heat exchanger, are assumed to manifest in the tabulated data.

Mixing Condition

Use the Cross flow arrangement parameter to mix each of the flows, one of the flows, or none of the flows. Mixing in this context is the lateral movement of fluid in channels that have no internal barriers, normally guides, baffles, fins, or walls. Such movement serves to even out temperature variations in the transverse plane. Mixed flows have variable temperature in the longitudinal plane alone. Unmixed flows have variable temperature in both the transverse and longitudinal planes. The figure shows a mixed flow (i) and an unmixed flow (ii).

The distinction between mixed and unmixed flows is considered only in cross flow arrangements. There, longitudinal temperature variation in one fluid produces transverse temperature variation in the second fluid that mixing can even out. In counter and parallel flow arrangements, longitudinal temperature variation in one fluid produces longitudinal temperature variation in the second fluid and mixing, as it is of little effect here, is ignored.

Effectiveness Curves

Shell-and-tube exchangers with multiple passes (iv.b-e in the figure for 2, 3, and 4 passes) are most effective. Of exchangers with a single pass, those with counter flows (ii are most effective and those with parallel flows (i) are least.

Cross-flow exchangers are intermediate in effectiveness, with mixing condition playing a factor. They are most effective when both flows are unmixed (iii.a) and least effective when both flows are mixed (iii.b). Mixing just the flow with the smallest heat capacity rate (iii.c) lowers the effectiveness more than mixing just the flow with the largest heat capacity rate (iii.d).

Thermal Resistance

The overall thermal resistance, R, is the sum of the local resistances lining the heat transfer path. The local resistances arise from convection at the surfaces of the wall, conduction through the wall, and, if the wall sides are fouled, conduction through the layers of fouling. Expressed in order from gas side to thermal liquid side:

R=1UGATh,G+FGATh,G+RW+FTLATh,TL+1UTLATh,TL,

where U is the convective heat transfer coefficient, F is the fouling factor, and ATh is the heat transfer surface area, each for the flow indicated in the subscript. RW is the thermal resistance of the wall.

The wall thermal resistance and fouling factors are simple constants obtained from block parameters. The heat transfer coefficients are elaborate functions of fluid properties, flow geometry, and wall friction, and derive from standard empirical correlations between Reynolds, Nusselt, and Prandtl numbers. The correlations depend on flow arrangement and mixing condition, and are detailed for each in the E-NTU Heat Transfer block on which the block model is based.

Thermal Mass

The wall is more than a thermal resistance for heat to pass through. It is also a thermal mass and, like the flows it divides, it can store heat in its bounds. The storage slows the transition between steady states so that a thermal perturbation on one side does not promptly manifest on the side across. The lag persists for the short time that it takes the heat flow rates from the two sides to balance each other. That time interval scales with the thermal mass of the wall:

CQ,W=cp,WMW,

where is the cp,W is the specific heat capacity and MW the inertial mass of the wall. Their product gives the energy required to raise wall temperature by one degree. Use the Wall thermal mass block parameter to specify that product. The parameter is active when you select Wall thermal dynamics.

Thermal mass is often negligible in low-pressure systems. Low pressure affords a thin wall with a transient response so fast that, on the time scale of the heat transfer, it is virtually instantaneous. The same is not true of high-pressure systems, common in the production of ammonia by the Haber process, where pressure can break 200 atmospheres. To withstand the high pressure, the wall is often thicker, and, as its thermal mass is heftier, so its transient response is slower.

Clear the Wall thermal dynamics checkbox to ignore the transient lag, cut the differential variables that produce it, and, in reducing calculations, speed up the rate of simulation. Leave it On to capture the transient lag where it has a measurable effect. Experiment with the setting if necessary to determine whether to account for thermal mass. If simulation results differ to a considerable degree, and if simulation speed is not a factor, keep the setting On.

The wall, if modeled with thermal mass, is considered in halves. One half sits on the gas side and the other half sits on the thermal liquid side. The gas side is denoted side 1 and the thermal liquid side is denoted side 2. This notation is used in the calculations for heat transfer. The thermal mass divides evenly between the pair:

CQ,1=CQ,2=CQ,W2.

Energy is conserved in the wall. In the simple case of a wall half at steady state, heat gained from the fluid equals heat lost to the second half. The heat flows at the rate predicted by the E-NTU method for a wall without thermal mass. The rate is positive for heat flows directed from side 1 of the heat exchanger to side 2:

Q1=Q2=ϵQMax.

In the transient state, the wall is in the course of storing or losing heat, and heat gained by one half no longer equals that lost to the second half. The difference in the heat flow rates varies over time in proportion to the rate at which the wall stores or loses heat. For side 1 of the heat exchanger:

Q1=ϵQMax+CQ,1T˙W,1,

where T˙W,1 is the rate of change in temperature in the wall half. Its product with the thermal mass of the wall half gives the rate at which heat accumulates there. That rate is positive when temperature rises and negative when it drops. The closer the rate is to zero the closer the wall is to steady state. For side 2 of the heat exchanger:Q2=ϵQMax+CQ,2T˙W,2,

Composite Structure

The block is a composite component built from simpler blocks. A Heat Exchanger Interface (G) block models the gas flow, a Heat Exchanger Interface (TL) block models the thermal liquid flow, and an E-NTU Heat Transfer block models the heat exchanged across the wall between the flows.

Ports

Conserving

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Opening for gas to enter and exit its side of the heat exchanger.

Opening for gas to enter and exit its side of the heat exchanger.

Opening for thermal liquid to enter and exit its side of the heat exchanger.

Opening for thermal liquid to enter and exit its side of the heat exchanger.

Parameters

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Common

Manner in which the flows align in the heat exchanger. The flows can run parallel to each other, counter to each other, or across each other. They can also run in a pressurized shell, one through tubes enclosed in the shell, the other around those tubes. Other flow arrangements are possible through a generic parameterization based on tabulated effectiveness data and requiring little detail about the heat exchanger.

Number of times the flow traverses the shell before exiting.

Dependencies

To enable this parameter, set Flow arrangement to Shell and tube.

Mixing condition in each of the flow channels. Mixing in this context is the lateral movement of fluid as it proceeds along its flow channel toward the outlet. The flows remain separate from each other. Unmixed flows are common in channels with plates, baffles, or fins. This setting reflects in the effectiveness of the heat exchanger, with unmixed flows being most effective and mixed flows being least.

Dependencies

To enable this parameter, set Flow arrangement to Shell and tube.

Number of transfer units at each breakpoint in the lookup table for the heat exchanger effectiveness number. The table is two-way, with both the number of transfer units and the thermal capacity ratio serving as independent coordinates. The block uses linear interpolation and nearest extrapolation to obtain the effectiveness at any number of transfer units.

The numbers specified must be greater than zero and increase monotonically from left to right. The size of the vector must equal the number of rows in the Effectiveness table parameter. If the table has m rows and n columns, the vector for the number of transfer units must be m elements long.

Dependencies

To enable this parameter, set Flow arrangement to Generic - effectiveness table.

Thermal capacity ratio at each breakpoint in lookup table for heat exchanger effectiveness. The table is two-way, with both the number of transfer units and the heat capacity rate ratio serving as independent coordinates. The block uses linear interpolation and nearest extrapolation to obtain the effectiveness at any thermal capacity ratio.

The thermal capacity ratios must be greater than zero and increase monotonically from left to right. The size of the vector must equal the number of columns in the Nusselt number table parameter. If the table has m rows and n columns, the vector for the thermal capacity ratio must be n elements long. The thermal capacity ratio is the fraction of minimum over maximum heat capacity rates.

Dependencies

To enable this parameter, set Flow arrangement to Generic - effectiveness table.

Heat exchanger effectiveness at each breakpoint in its lookup table over the number of transfer units and thermal capacity ratio. The block uses linear interpolation and nearest extrapolation to obtain the effectiveness at any pair of number of transfer units and thermal capacity ratio.

The effectiveness values must be not be negative. They must align from top to bottom in order of increasing number of transfer units and from left to right in order of increasing thermal capacity ratio. The number of rows must equal the size of the Number of heat transfer units vector parameter, and the number of columns must equal the size of the Thermal capacity ratio vector parameter.

Dependencies

To enable this parameter, set Flow arrangement to Generic - effectiveness table.

Modeling assumption for the transient response of the wall to thermal changes. Select Wall thermal dynamics to impart a thermal mass to the wall and to capture the delay in its transient response to changes in temperature or heat flux. Such delays are relevant in thick walls, such as those required to sustain high pressures. The default setting assumes a wall thin enough for its transient response to be virtually instantaneous on the time scale of the heat transfer.

Heat required to raise wall temperature by one degree. Thermal mass is the product of mass with specific heat and a measure of the ability to absorb heat. A wall with thermal mass has a transient response to sudden changes in surface temperature or heat flux. The larger the thermal mass, the slower that response, and the longer the time to steady state. The default value corresponds to a wall of stainless steel with a mass of approximately 1 kg.

Dependencies

To enable this parameter, select Wall thermal dynamics.

Resistance of the wall to heat flow by thermal conduction. Wall resistance adds to convective and fouling resistances to determine the overall heat transfer coefficient between the flows.

Gas 1

Total area of the flow cross section measured where the channel is its narrowest and the flow its fastest. If the channel is a collection of ducts, tubes, slots, or grooves, the area is the sum over the collection (minus any occlusion due to walls, ridges, plates, and other barriers).

Effective inner diameter of the flow at its narrowest point. For channels not circular in cross section, that diameter is of an imaginary circle equal in area to the flow cross section. Its value is the ratio of the minimum free-flow area to a fourth of its gross perimeter.

If the channel is a collection of ducts, tubes, slots, or grooves, the gross perimeter is the sum of the perimeters in the collection. If the channel is a single pipe or tube and it is circular in cross section, the hydraulic diameter is the same as the true diameter.

Total volume of fluid contained in the gas flow channel.

Start of transition between laminar and turbulent zones. Above this number, inertial forces take hold and the flow grows progressively turbulent. The default value is characteristic of circular pipes and tubes with smooth surfaces.

End of transition between laminar and turbulent zones. Below this number, viscous forces take hold and the flow grows progressively laminar. The default value is characteristic of circular pipes and tubes with smooth surfaces.

Mathematical model for pressure loss by viscous friction. This setting determines which expressions to use for calculation and which block parameters to specify as input. See the Heat Exchanger Interface (G) and Heat Exchanger Interface (TL) blocks for the calculations by parameterization.

Aggregate loss coefficient for all flow resistances in the flow channel including the wall friction responsible for major loss and the local resistances, due to bends, elbows, and other geometry changes, responsible for minor loss.

The loss coefficient is an empirical dimensionless number commonly used to express the pressure loss due to viscous friction. It can be calculated from experimental data or, in some cases, obtained from product data sheets.

Dependencies

To enable this parameter, set Pressure loss model to Pressure loss coefficient.

Total distance the flow must travel to reach across the ports. In multi-pass shell-and-tube exchangers, the total distance is the sum over all shell passes. In tube bundles, corrugated plates, and other channels in which the flow is split into parallel branches, it is the distance covered in a single branch. The longer the flow path, the steeper the major pressure loss due to viscous friction at the wall.

Dependencies

To enable this parameter, set Pressure loss model to Correlation for flow inside tubes or Tabulated data - Darcy friction factor vs Reynolds number.

Aggregate minor pressure loss expressed as a length. This length is that which all local resistances, such as elbows, tees, and unions, would add to the flow path if in their place was a simple wall extension. The larger the equivalent length, the steeper the minor pressure loss due to the local resistances.

Dependencies

To enable this parameter, set Pressure loss model to Correlation for flow inside tubes.

Mean height of the surface protrusions from which wall friction arises. Higher protrusions mean a rougher wall for more friction and so a steeper pressure loss. Surface roughness features in the Haaland correlation from which the Darcy friction factor derives and on which the pressure loss calculation depends.

Dependencies

To enable this parameter, set Pressure loss model to Correlation for flow inside tubes.

Pressure loss correction for flow cross section in laminar flow conditions. This parameter is commonly referred to as the shape factor. Its ratio to the Reynolds number gives the Darcy friction factor for the pressure loss calculation in the laminar zone. The default value belongs to cylindrical pipes and tubes.

The shape factor derives for certain shapes from the solution of the Navier-Stokes equations. A square duct has a shape factor of 56, a rectangular duct with aspect ratio of 2:1 has a shape factor of 62, and an annular tube has a shape factor of 96, as does a slender conduit between parallel plates.

Dependencies

To enable this parameter, set Pressure loss model to Correlation for flow inside tubes.

Reynolds number at each breakpoint in the lookup table for the Darcy friction factor. The block uses linear interpolation and nearest extrapolation to obtain the Darcy friction factor at any Reynolds number.

The Reynolds numbers must be greater than zero and increase monotonically from left to right. They can span across laminar, transient, and turbulent zones. Their number must equal the size of the Darcy friction factor vector parameter, with which they are to combine to complete the tabulated breakpoints.

Dependencies

To enable this parameter, set Pressure loss model to Tabulated data - Darcy friction factor vs. Reynolds number.

Darcy friction factor at each breakpoint in its lookup table over the Reynolds number. The block uses linear interpolation and nearest extrapolation to obtain the Darcy friction factor at any Reynolds number.

The Darcy friction factors must not be negative and they must align from left to right in order of increasing Reynolds number. Their number must equal the size of the Reynolds number vector for Darcy friction factor parameter, with which they are to combine to complete the tabulated breakpoints.

Dependencies

To enable this parameter, set Pressure loss model to Tabulated data - Darcy friction factor vs. Reynolds number.

Reynolds number at each breakpoint in the lookup table for the Euler number. The block uses linear interpolation and nearest extrapolation to obtain the Euler number at any Reynolds number.

The Reynolds numbers must be greater than zero and increase monotonically from left to right. They can span across laminar, transient, and turbulent zones. Their number must equal the size of the Euler number vector parameter, with which they are to combine to complete the tabulated breakpoints.

Dependencies

To enable this parameter, set Pressure loss model to Tabulated data - Euler number vs. Reynolds number.

Euler number at each breakpoint in its lookup table over the Reynolds number. The block uses linear interpolation and nearest extrapolation to obtain the Euler number at any Reynolds number.

The Euler numbers must not be negative and they must align from left to right in order of increasing Reynolds number. Their number must equal the size of the Reynolds number vector for Euler number parameter, with which they are to combine to complete the tabulated breakpoints.

Dependencies

To enable this parameter, set Pressure loss model to Tabulated data - Euler number vs. Reynolds number.

Mathematical model for heat transfer between fluid and wall. The choice of model determines which expressions to apply and which parameters to specify for heat transfer calculation. See the E-NTU Heat Transfer block for the calculations by parameterization.

Effective surface area used in heat transfer between fluid and wall. The effective surface area is the sum of primary and secondary surface areas, or those of the wall, where it is exposed to fluid, and of the fins, if any are used. Fin surface area is normally scaled by a fin efficiency factor.

Heat transfer coefficient for convection between gas and wall. Resistance due to fouling is captured separately in the Fouling factor parameter.

Dependencies

To enable this parameter, set Heat transfer coefficient model to Constant heat transfer coefficient.

Characteristic length traversed in heat transfer between fluid and wall. This length factors in the calculation of the hydraulic diameter from which the heat transfer coefficient and the Reynolds number, as defined in the tabulated heat transfer parameterizations, derives.

Dependencies

To enable this parameter, set Heat transfer coefficient model to Tabulated data - Colburn factor vs. Reynolds number or Tabulated data - Nusselt number vs. Reynolds number and Prandtl number.

Constant assumed for Nusselt number in laminar flow. The Nusselt number factors in the calculation of the heat transfer coefficient between fluid and wall, on which the heat transfer rate depends. The default value belongs to cylindrical pipes and tubes.

Dependencies

To enable this parameter, set Heat transfer coefficient model to Correlation for flow inside tubes.

Reynolds number at each breakpoint in the lookup table for the Colburn factor. The block uses linear interpolation and nearest extrapolation to obtain the Colburn factor at any Reynolds number.

The Reynolds numbers must be greater than zero and increase monotonically from left to right. They can span across laminar, transient, and turbulent zones. Their number must equal the size of the Colburn factor vector parameter, with which they are to combine to complete the tabulated breakpoints.

Dependencies

To enable this parameter, set Heat transfer coefficient model to Tabulated data - Colburn factor vs. Reynolds number.

Colburn factor at each breakpoint in its lookup table over the Reynolds number. The block uses linear interpolation and nearest extrapolation to obtain the Euler number at any Reynolds number.

The Colburn factors must not be negative and they must align from left to right in order of increasing Reynolds number. Their number must equal the size of the Reynolds number vector for Colburn factor parameter, with which they are to combine to complete the tabulated breakpoints.

Dependencies

To enable this parameter, set Heat transfer coefficient model to Tabulated data - Colburn factor vs. Reynolds number.

Reynolds number at each breakpoint in the lookup table for the Nusselt number. The table is two-way, with both Reynolds and Prandtl numbers serving as independent coordinates. The block uses linear interpolation and nearest extrapolation to obtain the Nusselt number at any Reynolds number.

The Reynolds numbers must be greater than zero and increase monotonically from left to right. They can span across laminar, transient, and turbulent zones. The size of the vector must equal the number of rows in the Nusselt number table parameter. If the table has m rows and n columns, the Reynolds number vector must be m elements long.

Dependencies

To enable this parameter, set Heat transfer coefficient model to Tabulated data - Nusselt number vs. Reynolds number and Prandtl number.

Prandtl number at each breakpoint in the lookup table for the Nusselt number. The table is two-way, with both Reynolds and Prandtl numbers serving as independent coordinates. The block uses linear interpolation and nearest extrapolation to obtain the Nusselt number at any Prandtl number.

The Prandtl numbers must be greater than zero and increase monotonically from left to right. They can span across laminar, transient, and turbulent zones. The size of the vector must equal the number of columns in the Nusselt number table parameter. If the table has m rows and n columns, the Prandtl number vector must be n elements long.

Dependencies

To enable this parameter, set Heat transfer coefficient model to Tabulated data - Nusselt number vs. Reynolds number and Prandtl number.

Nusselt number at each breakpoint in its lookup table over the Reynolds and Prandtl numbers. The block uses linear interpolation and nearest extrapolation to obtain the Nusselt number at any pair of Reynolds and Prandtl numbers. By determining the Nusselt number, the table feeds the calculation from which the heat transfer coefficient between fluid and wall derives.

The Nusselt numbers must be greater than zero. They must align from top to bottom in order of increasing Reynolds number and from left to right in order of increasing Prandtl numbers. The number of rows must equal the size of the Reynolds number vector for Nusselt number parameter, and the number of columns must equal the size of the Prandtl number vector for Nusselt number parameter.

Dependencies

To enable this parameter, set Heat transfer coefficient model to Tabulated data - Nusselt number vs. Reynolds number and Prandtl number.

Measure of thermal resistance due to fouling deposits which over time tend to build on the exposed surfaces of the wall. The deposits, as they impose between the fluid and wall a new solid layer through which heat must traverse, add to the heat transfer path an extra thermal resistance. Fouling deposits grow slowly and the resistance due to them is accordingly assumed constant during simulation.

Lower bound for the heat transfer coefficient between fluid and wall. If calculation returns a lower heat transfer coefficient, this bound replaces the calculated value.

Thermal Liquid 2

Total area of the flow cross section measured where the channel is its narrowest and the flow its fastest. If the channel is a collection of ducts, tubes, slots, or grooves, the area is the sum over the collection (minus any occlusion due to walls, ridges, plates, and other barriers).

Effective inner diameter of the flow at its narrowest point. For channels not circular in cross section, that diameter is of an imaginary circle equal in area to the flow cross section. Its value is the ratio of the minimum free-flow area to a fourth of its gross perimeter.

If the channel is a collection of ducts, tubes, slots, or grooves, the gross perimeter is the sum of the perimeters in the collection. If the channel is a single pipe or tube and it is circular in cross section, the hydraulic diameter is the same as the true diameter.

Total volume of fluid contained in the thermal liquid flow channel.

Start of transition between laminar and turbulent zones. Above this number, inertial forces take hold and the flow grows progressively turbulent. The default value is characteristic of circular pipes and tubes with smooth surfaces.

End of transition between laminar and turbulent zones. Below this number, viscous forces take hold and the flow grows progressively laminar. The default value is characteristic of circular pipes and tubes with smooth surfaces.

Mathematical model for pressure loss by viscous friction. This setting determines which expressions to use for calculation and which block parameters to specify as input. See the Heat Exchanger Interface (G) and Heat Exchanger Interface (TL) blocks for the calculations by parameterization.

Aggregate loss coefficient for all flow resistances in the flow channel including the wall friction responsible for major loss and the local resistances, due to bends, elbows, and other geometry changes, responsible for minor loss.

The loss coefficient is an empirical dimensionless number commonly used to express the pressure loss due to viscous friction. It can be calculated from experimental data or, in some cases, obtained from product data sheets.

Dependencies

To enable this parameter, set Pressure loss model to Pressure loss coefficient.

Total distance the flow must travel to reach across the ports. In multi-pass shell-and-tube exchangers, the total distance is the sum over all shell passes. In tube bundles, corrugated plates, and other channels in which the flow is split into parallel branches, it is the distance covered in a single branch. The longer the flow path, the steeper the major pressure loss due to viscous friction at the wall.

Dependencies

To enable this parameter, set Pressure loss model to Correlation for flow inside tubes or Tabulated data - Darcy friction factor vs Reynolds number.

Aggregate minor pressure loss expressed as a length. This length is that which all local resistances, such as elbows, tees, and unions, would add to the flow path if in their place was a simple wall extension. The larger the equivalent length, the steeper the minor pressure loss due to the local resistances.

Dependencies

To enable this parameter, set Pressure loss model to Correlation for flow inside tubes.

Mean height of the surface protrusions from which wall friction arises. Higher protrusions mean a rougher wall for more friction and so a steeper pressure loss. Surface roughness features in the Haaland correlation from which the Darcy friction factor derives and on which the pressure loss calculation depends.

Dependencies

To enable this parameter, set Pressure loss model to Correlation for flow inside tubes.

Pressure loss correction for flow cross section in laminar flow conditions. This parameter is commonly referred to as the shape factor. Its ratio to the Reynolds number gives the Darcy friction factor for the pressure loss calculation in the laminar zone. The default value belongs to cylindrical pipes and tubes.

The shape factor derives for certain shapes from the solution of the Navier-Stokes equations. A square duct has a shape factor of 56, a rectangular duct with aspect ratio of 2:1 has a shape factor of 62, and an annular tube has a shape factor of 96, as does a slender conduit between parallel plates.

Dependencies

To enable this parameter, set Pressure loss model to Correlation for flow inside tubes.

Reynolds number at each breakpoint in the lookup table for the Darcy friction factor. The block uses linear interpolation and nearest extrapolation to obtain the Darcy friction factor at any Reynolds number.

The Reynolds numbers must be greater than zero and increase monotonically from left to right. They can span across laminar, transient, and turbulent zones. Their number must equal the size of the Darcy friction factor vector parameter, with which they are to combine to complete the tabulated breakpoints.

Dependencies

To enable this parameter, set Pressure loss model to Tabulated data - Darcy friction factor vs. Reynolds number.

Darcy friction factor at each breakpoint in its lookup table over the Reynolds number. The block uses linear interpolation and nearest extrapolation to obtain the Darcy friction factor at any Reynolds number.

The Darcy friction factors must not be negative and they must align from left to right in order of increasing Reynolds number. Their number must equal the size of the Reynolds number vector for Darcy friction factor parameter, with which they are to combine to complete the tabulated breakpoints.

Dependencies

To enable this parameter, set Pressure loss model to Tabulated data - Darcy friction factor vs. Reynolds number.

Reynolds number at each breakpoint in the lookup table for the Euler number. The block uses linear interpolation and nearest extrapolation to obtain the Euler number at any Reynolds number.

The Reynolds numbers must be greater than zero and increase monotonically from left to right. They can span across laminar, transient, and turbulent zones. Their number must equal the size of the Euler number vector parameter, with which they are to combine to complete the tabulated breakpoints.

Dependencies

To enable this parameter, set Pressure loss model to Tabulated data - Euler number vs. Reynolds number.

Euler number at each breakpoint in its lookup table over the Reynolds number. The block uses linear interpolation and nearest extrapolation to obtain the Euler number at any Reynolds number.

The Euler numbers must not be negative and they must align from left to right in order of increasing Reynolds number. Their number must equal the size of the Reynolds number vector for Euler number parameter, with which they are to combine to complete the tabulated breakpoints.

Dependencies

To enable this parameter, set Pressure loss model to Tabulated data - Euler number vs. Reynolds number.

Mathematical model for heat transfer between fluid and wall. The choice of model determines which expressions to apply and which parameters to specify for heat transfer calculation. See the E-NTU Heat Transfer block for the calculations by parameterization.

Effective surface area used in heat transfer between fluid and wall. The effective surface area is the sum of primary and secondary surface areas, or those of the wall, where it is exposed to fluid, and of the fins, if any are used. Fin surface area is normally scaled by a fin efficiency factor.

Heat transfer coefficient for convection between fluid and wall. Resistance due to fouling is captured separately in the Fouling factor parameter.

Dependencies

To enable this parameter, set Heat transfer coefficient model to Constant heat transfer coefficient.

Characteristic length traversed in heat transfer between fluid and wall. This length factors in the calculation of the hydraulic diameter from which the heat transfer coefficient and the Reynolds number, as defined in the tabulated heat transfer parameterizations, derives.

Dependencies

To enable this parameter, set Heat transfer coefficient model to Tabulated data - Colburn factor vs. Reynolds number or Tabulated data - Nusselt number vs. Reynolds number and Prandtl number.

Constant assumed for Nusselt number in laminar flow. The Nusselt number factors in the calculation of the heat transfer coefficient between fluid and wall, on which the heat transfer rate depends. The default value belongs to cylindrical pipes and tubes.

Dependencies

To enable this parameter, set Heat transfer coefficient model to Correlation for flow inside tubes.

Reynolds number at each breakpoint in the lookup table for the Colburn factor. The block uses linear interpolation and nearest extrapolation to obtain the Colburn factor at any Reynolds number.

The Reynolds numbers must be greater than zero and increase monotonically from left to right. They can span across laminar, transient, and turbulent zones. Their number must equal the size of the Colburn factor vector parameter, with which they are to combine to complete the tabulated breakpoints.

Dependencies

To enable this parameter, set Heat transfer coefficient model to Tabulated data - Colburn factor vs. Reynolds number.

Colburn factor at each breakpoint in its lookup table over the Reynolds number. The block uses linear interpolation and nearest extrapolation to obtain the Euler number at any Reynolds number.

The Colburn factors must not be negative and they must align from left to right in order of increasing Reynolds number. Their number must equal the size of the Reynolds number vector for Colburn factor parameter, with which they are to combine to complete the tabulated breakpoints.

Dependencies

To enable this parameter, set Heat transfer coefficient model to Tabulated data - Colburn factor vs. Reynolds number.

Reynolds number at each breakpoint in the lookup table for the Nusselt number. The table is two-way, with both Reynolds and Prandtl numbers serving as independent coordinates. The block uses linear interpolation and nearest extrapolation to obtain the Nusselt number at any Reynolds number.

The Reynolds numbers must be greater than zero and increase monotonically from left to right. They can span across laminar, transient, and turbulent zones. The size of the vector must equal the number of rows in the Nusselt number table parameter. If the table has m rows and n columns, the Reynolds number vector must be m elements long.

Dependencies

To enable this parameter, set Heat transfer coefficient model to Tabulated data - Nusselt number vs. Reynolds number and Prandtl number.

Prandtl number at each breakpoint in the lookup table for the Nusselt number. The table is two-way, with both Reynolds and Prandtl numbers serving as independent coordinates. The block uses linear interpolation and nearest extrapolation to obtain the Nusselt number at any Prandtl number.

The Prandtl numbers must be greater than zero and increase monotonically from left to right. They can span across laminar, transient, and turbulent zones. The size of the vector must equal the number of columns in the Nusselt number table parameter. If the table has m rows and n columns, the Prandtl number vector must be n elements long.

Dependencies

To enable this parameter, set Heat transfer coefficient model to Tabulated data - Nusselt number vs. Reynolds number and Prandtl number.

Nusselt number at each breakpoint in its lookup table over the Reynolds and Prandtl numbers. The block uses linear interpolation and nearest extrapolation to obtain the Nusselt number at any pair of Reynolds and Prandtl numbers. By determining the Nusselt number, the table feeds the calculation from which the heat transfer coefficient between fluid and wall derives.

The Nusselt numbers must be greater than zero. They must align from top to bottom in order of increasing Reynolds number and from left to right in order of increasing Prandtl numbers. The number of rows must equal the size of the Reynolds number vector for Nusselt number parameter, and the number of columns must equal the size of the Prandtl number vector for Nusselt number parameter.

Dependencies

To enable this parameter, set Heat transfer coefficient model to Tabulated data - Nusselt number vs. Reynolds number and Prandtl number.

Measure of thermal resistance due to fouling deposits which over time tend to build on the exposed surfaces of the wall. The deposits, as they impose between the fluid and wall a new solid layer through which heat must traverse, add to the heat transfer path an extra thermal resistance. Fouling deposits grow slowly and the resistance due to them is accordingly assumed constant during simulation.

Lower bound for the heat transfer coefficient between fluid and wall. If calculation returns a lower heat transfer coefficient, this bound replaces the calculated value.

Effects and Initial Conditions

Temperature in the gas channel at the start of simulation

Pressure in the gas channel at the start of simulation.

Option to model the pressure dynamics in the thermal liquid channel. If you clear this checkbox, the block removes the pressure derivative terms from the component energy and mass conservation equations. The pressure inside the heat exchanger is then reduced to the weighted average of the two port pressures.

Temperature in the thermal liquid channel at the start of simulation

Pressure in the thermal liquid channel at the start of simulation.

Extended Capabilities

C/C++ Code Generation
Generate C and C++ code using Simulink® Coder™.

Version History

Introduced in R2019a

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