Page 477 - Design and Operation of Heat Exchangers and their Networks

460 Appendix

Example 2.3 Cooling of a printed circuit board
(MatLab code)

% Example 2.3 Cooling of a printed circuit board
% A printed circuit board is cooled by blowing air through a heat sink as
% is shown in Fig. 2.6. The printed circuit board is 150 mm in length and
% 80 mm in width and has a heat duty of 100W. The heat sink is made of
% aluminum and has 13 rectangular air flow channels with channel spacing
% of 25 mm in height, channel spacing of 3.5 mm in width and wall thickness
% of 2.5 mm. The thermal conductivity of the heat sink material is 230 W/mK.
% The air flow rate is 18 m3/h at the inlet air temperature of 25°C.
% Assuming that the heat flux is uniform over the printed circuit board,
% evaluate its highest temperature.
lambda_f = 230; % thermal conductivity of fins, W/mK
L = 0.15; % length of the board, m
B = 0.08; % width of the board, m
Q = 100; % heat duty, W
delta = 0.0025; % wall thickness, m
delta_f = 0.0025; % wall thickness, m
h_fs = 0.025; % channel height, m
s_fs = 0.0035; % channel spacing, m
N = 13; % number of channels
B_c = 0.0805; % width of the cooler, m
t_in = 25; %inlet air temperature, °C
p_in = 1; % inlet air pressure, bar
V = 18; % air flow rate, m3/h
rho_in = refpropm('D','T', t_in + 273.15, 'P', p_in ∗ 100, 'air');
% density at inlet, kg/m3
m = rho_in ∗ V / 3600; % mass flow rate, kg/s
G =m/(N ∗ h_fs ∗ s_fs); % mass velocity, kg/m2s
dh = 2 ∗ h_fs ∗ s_fs / (h_fs + s_fs); % hydraulic diameter, m
gamma = s_fs / h_fs; % channel width to height ratio
t_m = t_in; % assumed mean temperature, °C
cp = refpropm('C','T', t_m + 273.15, 'P', p_in ∗ 100, 'air');
% isobaric heat capacity, J/kgK
t_out = t_in +Q/m/cp; % calculated outlet air temperature, °C
t_m = (t_in + t_out) / 2; % mean temperature, °C
[rho, mu, lambda, cp] = refpropm('DVLC','T', t_m + 273.15, ...
'P', p_in ∗ 100, 'air');