Fluid temperature, average

For the average fluid temperature and/or true caloric temperature see Temperature for Fluid Properties Evaluation—Caloric Temperature later in this chapter. 

Figure 10-38. Caloric or true average fluid temperature. (Used by permission Standards of Tubular Exchanger Manufacturers Association, 1959 and 1968. Tubular Exchanger Manufacturers Association, Inc. All rights reserved.)...

Follow steps 7 (Gilmour method), etc., of the procedure for vaporization only. If baffles are added for sensible heat (not assumed in free convection), then pressure drop will be affected accordingly. Gr is the Grashof number using properties at average fluid temperature, = Dj pgP At/p. ... 

To estimate the value of the term on the left-hand side of Eq. (4.4), we use an approximate expression for the local and average fluid temperature in the tube. We use... 

It should be emphasized that under conditions of energy dissipation the definition of the heat transfer coefficient as k dT/dr)/ T — T ), where T is the average fluid temperature and 7 is the wall temperature, does not characterize the acmal heat transfer properly (Kays and Crawford 1993 Schlichfing 2000). 

Assuming that d 0i/ dX +2 sj d202/dX+2 the following equation for average fluid temperature was obtained... 

Average fluid temperature Average velocity Normalized local velocity Streamwise velocity... 

The pressure drop fluctuation provides insight into the temperature behavior of the fluid in the outlet manifold. The pressure drop fluctuation frequency is representative of the oscillations in the system. Figure 6.38a,b shows time variation and FFT of the fluctuation component of the fluid temperature. From Fig. 6.38a one can see that the average fluid temperature at the outlet manifold is less than the saturation temperature. This results in the fact that only single liquid comes to the outlet manifold through some of the parallel micro-channels. 

This means that the average fluid temperature Tm(z) in the thermally developed region increases linearly with z. Substituting in Eq. (8.44) and... 

We assume that the heat flux at the wall is constant, so that the average fluid temperature must increase linearly with x, or... 

Discussion The average fluid temperature is (80 + /1.3)/2 = /b./°C, which is sufficiently close to 80°C at which we evaluated the properties of air. Therefore, it is not necessary to re evaluate the properties at this temperature and to repeal the calculations. 

Since the ratio of surface area to volume is large, viscous heating is an important factor in microchannels. It is especially important for laminar flow, where considerable gradients exist. The Brinkman number, Br, indicates this effect. A decrease in Nu for Br > 0 and an increase for Br < 0 have been observed. This is due to the fact that for different cases, Br may increase or decrease the driving mechanism for convective heat transfer, which is the difference between the wall temperature and the average fluid temperature. 

In comparing the performance of a real cycle with that of a Carnot cycle, one has in principle a choice of temperatures to use for the Carnot calculation. Consider a vapor-compression refrigeration cycle in which tlie average fluid temperatures m the condenser and evaporator are Th aiid Tc, respectively. Corresponding to Th and Tc, tlie heat transfer occurs with respect to surroundings at temperature T , and Which provides the more conservative estimate of j(Camot) a calculation based on Th and Tc, or one based on Ts and... 

Fig. 1.8 Temperature profile in a channel cross section. Wall temperature and average fluid temperature i F...

The flow of heat causes a change in the enthalpy flow H of the fluid. The cross sectional average fluid temperature i F is now defined such that the enthalpy flow can be written as... 

Fig. 1.10 Average fluid temperature i p, wall temperature and local heat transfer coefficient as functions of the axial distance z, when heating a fluid in a tube of length L...

To determine am experimentally only M, the wall temperature and average fluid temperature at entry and exit need to be measured. The use of the logarithmic mean value is also suggested by the following when both the wall temperature and the local heat transfer coefficient a are constant, then am = a is only true when A og is used in the definition for am. Then from (1.32) it follows that... 

Normal heaters will handle about 70% of the total duty in the radiant section. On this basis, estimate the radiant heat absorption Qr (Btu per hour). From enthalpy or specific heat data for the process fluid, estimate the temperature at the crossover between the convection and radiant sections and the average fluid temperature in the... 

Using these temperatures, calculate the average fluid temperature, the log mean temperature difference, the average gas temperature and the average gas film temperature. 

For flowing cooling water, a fouling factor of m = Viscosity at average fluid temperature. 

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