Determination of the area required for heat transfer in exchangers or the mass transfer area in mass exchange units requires the determination of transfer coefficients. We will describe some programs that should be helpful in these calculations. A first set of programs was written by Jim Lee in 1995 to estimate physical properties of low density gases. In heat exchanger design, we primarily need to know the viscosity and thermal conductivity of the fluids involved. In mass transfer operations, we need to calculate diffusivities. Programs users might need in order to determine these properties are listed in the following table.
|
Program |
Determines |
Assumptions |
|---|---|---|
|
Mass diffusivity |
Binary mixture at low density |
|
|
Thermal Conductivity of monatomic gases |
Low density monatomic gas. |
|
|
Thermal Conductivity of polyatomic gases |
Polyatomic gas at low density |
|
|
Thermal conductivity of gas mixture |
Gas at low density |
|
|
Viscosity of gas mixture |
Gas at low density |
|
|
mixphi |
Phi array of BS&L eq. 1.4-20 |
Gases at low densities |
|
Viscosities of compound(s) at one temperature |
Gas at low density |
|
|
muscalc |
Viscosities of compound(s) at several temperatures |
Gases at low density |
|
Omega given in Table B-2 of BS&L |
Gas at low density |
|
|
Prandtl number of gases |
Gas at low density |
NB: The mixphi and muscalc programs are primarily useful in finding parameters for the other programs. Thus we will not demonstrate them although they will be used. Before any of these programs can be executed, compounds must be identified and data set for them in Matlab arrays. The Matlab function start301 may be used to set the required arrays. We will use it to set data for carbon dioxide, oxygen, nitrogen, and neon.
»start301 Copyright 1998 Rice University All rights reserved Welcome to CENG301's start301!! You have three choices, Please read them carefully Then click on the appropriate choice in the menu bar 1: Click (1) to start a new session <-- We did this 2: Click (2) to enter the name of a datafile from a previous session, in the current directory 3: Click (3) to use compound names not in the CENG 301 database WARNING...if you hit (3) no compound data will be available Now choose from the above menu <-- We chose the ceng301 data base Please wait while loading database... Which units do you want to use for temperature? <-- We chaos K Choose one from the menu Input the name of your new file: t402 The output file name is: t402 Input the number of compounds: 4 The number of compounds is: 4 Enter the name of compound # 1: CO2 Enter the name of compound # 2: O2 Enter the name of compound # 3: N2 Enter the name of compound # 4: Ne cnms = carbon dioxide oxygen nitrogen neon form = CO2 O2 N2 Ne Enter the number of reactions: 0 Here are your compounds' formulae and names: No. Formula Name ---------------------------------------- 1 CO2 carbon dioxide 2 O2 oxygen 3 N2 nitrogen 4 Ne neon Here are your reactions: ---------------------------------------- No reactions given Enter the number of streams: 0 The variables for your compounds have now been created, you may continue, or come back later and reload the same data. »
The transport property programs use Lennard-Jones parameters to determine all three transport coefficients. For all four of our compounds these parameters are stored in the Matlab array:
>>LJones
LJones =
3.9960 190.0000
3.4330 113.0000
3.6810 91.5000
2.7890 35.7000
Here is a listing of the function omegacalc:
The function omegacalc uses spline fits of the data in Table B-2 of BS&L to
estimate the values of omega used in finding transport properties for ideal gases.
function [OMEGA_MU, OMEGA_K, OMEGA_D] = omegacalc (x)
% function [omu,oK,oD]=omegacalc(x)
% Calculates a value for omega. Three values of omega are returned:
% [viscosity, thermal conductivity, mass diffusivity]
% Based on functions shown in Table B-2 of BS&L
% Argument List:
% x value of kappa * T / epsilon where:
% kappa/epsilon is a Lennard-Jones parameter of the substance and
% T is the temperature in Kelvin
%
% Ex: >> start402b or (global ppsp followed by load ppsp)
% >> [omega_mu, omega_k, omega_D] = omegacalc(0.30)
% omega_mu =
% 2.7850
% omega_k =
% 2.7850
% omega_D =
% 2.6620
% Created 3/16/95-4 by Jim Lee (revised several times)
global ppsp
= ppval(ppsp(:,1),x);
= OMEGA_MU;
= ppval(ppsp(:,2),x);
% This file was created by m2html.
The comments in the function describe the use of the function. The command ppsp evaluates a "piecewise" polynomial that was created using splines for the data in Table B-2 of BS&L.