View/Open

Ca
CaSt Ca Ca
(3.56)
Q m c
58
dT
dt
3.4.6 Heat transfer from chamber air into ambient via wall 2 and 3
The flow pattern through the chamber is very complicated. The situation is simplified
by assuming that the air flow in the chamber flows only once over all the chamber inner
surfaces [19]. It is further assumed that the flow produces a characteristic velocity over
all the surfaces which is defined by Eq. (3.34) as u
C
q
C
0.5A
Note that the water surface is dropping while the CPAP is in use due to evaporation.
However neglecting this change will not influence the objectives of the project. Also
due to flow velocity may range widely from 0 to 100 L/min., both natural and forced
convections need to be included. To calculate T C 23i
, the temperature at the inner surface
of wall 2 and wall 3, energy balance at the inner surface is carried out below.
The heat transfer toward the chamber wall 2 and 3 at their inner surfaces is converted to
wall heat storage and heat dissipation into ambient:
C 23i C 23St C23o Co
Q Q Q
(3.57)
Since the breathing induced airflow fluctuating is very short comparing to the chamber
wall temperature change, the wall 2 and wall 3 temperature is simplified as constant
therefore QC 23St
is neglected. Thus Eq. (3.57) can be changed as:
TCa TC 23i TC 23o
T
(3.58)
R R
C 23i C23o Same as the analysis of wall 1 thermal balance, inner surface temperature of wall 2 and
3 can be calculated as:
T
C23i 1 1 T T
R R R R R R
1 Ca
( ) ( )
C 23i C 23 C 23o C23i C23 C23o .
(3.59)
Where RC 23 is the conductive thermal resistance of wall 2 and wall 3 together. Heat
transfer through the walls includes mixed convection at inner surface, conduction