Commensurate units error - Simscape Covective heat transfer (Pipe)

Hi all,

I'm trying to create a custom block in simulink for external convective heat transfer for a cylinder. The equation for convective heat transfer coefficient uses a couple of empirical coefficients which are just numbers and do not have units (see attached image for equation). I am getting an error in the code of 'uncomensurate units' (see attached images) i believe because these coefficients need to be physical values.

From pervious models i understand that coefficients of an equation units need to be comensurate which i have tried to do here but not sure why i am getting the error. Any advice would be apprecitated

component convection_radial_empirical < pipe_model.branchThermal
% Convective Heat Transfer - Radial Empirical Correlation
% The block represents an energy transfer by convection between two
% bodies by means of fluid motion. The transfer is governed by the Newton
% law of cooling and is directly proportional to the convection heat 
% transfer coefficient, pipe surface area and the temperature difference.
%
% Connections A and B are thermal conserving ports associated
% with the points between which the energy transport by convection takes
% place. The block positive direction is from port A to port B. This means
% that the heat flow is positive if it flows from A to B. 

% Copyright 2005-2016 The MathWorks, Inc.

parameters
    d = { 1e-4, 'm' }; % Outer diameter
    l = { 1e-4, 'm' }; % Length
    C1 = { 1e-4, 'm' }; % Shape factor coefficient
    Airflow = { 1, 'm/s^3' }; % External airflow
end

parameters (Access=private)
    a = { 1, 'm/s' }; % C1 - Number 1
    b = { 1.277, 'm/s^2' }; % C1 - Number 2
end
    
variables
    heat_tr_coeff = { 20, 'W/(m^2*K)' }; % Heat transfer coefficient
    C2 = { 1, 'm/s' }; % Airflow coefficient 2
    
end

equations
    assert(d>0)
    assert(l>0)
    assert(heat_tr_coeff>0)

    C2 == sqrt(a + b * Airflow)
    heat_tr_coeff == C1 *((1 / d)^0.2) * (1 / (Tavg))^0.181 * (T)^0.266 * C2
    Q == heat_tr_coeff * pi * d * (T) * l;
end

end

Answer 1

[Yifeng Tang]

Hi Aled,

Consider use the syntax

value(a,'unit','type')

to use only the value of a variable with specified unit.

More details here:

https://www.mathworks.com/help/physmod/simscape/lang/value.html