Heat exchanger linear velocity

In the PCU and primary system, however, there is only one compressor to manage mass flow rate while there are several different circuits. To achieve the desired control of helium mass flow rate compressor control provides little flexibility. Rather inventory control is used to obtain a flow rate proportional to heat exchanger power. Because density is proportional to pressure for fixed temperature, by varying pressure and maintaining constant speed turbo-machinery, gas velocity remains constant and mass flow rate (proportional to the product of density and velocity) is linear with pressure. Thus, pressure is manipulated through coolant mass inventory so that it is proportional to heat exchanger power so that in turn mass flow rate is proportional to heat exchanger power. Results obtained for this control scheme are described below. 

In a heat exchanger, air flows through a pipe of length, l. The air enters the pipe at a temperature, Tt. The heat flux at the wall of this pipe increases linearly from zero at the inlet of the pipe to a value of qw at the end of the pipe. The velocity in the pipe is such that HRe PrD — 0.07 where D is the diameter of the pipe and Re is the Reynolds number. Determine how the Nusselt number based on the local wall heat transfer rate and on the difference between the local wall temperature and the inlet temperature varies with the dimensionless distance. Z, along the pipe. 

Let us have some perturbed state of a body made from the linear fluid (3.187)-(3.196) with regular equilibrium response ((3.232) is valid) and with stability conditions (3.256), (3.257), which is held permanently in an inertial frame without body force in isolation (no heat, work and mass exchange with surroundings). That is we have persistently through the body i = o (3.48), b = o, no heat radiation Q = 0 (3.231) and on its boundary no heat exchange q = o and zero velocity v = o. 

From a physical point of view, it seems that measurable quantities are mixture invariant (cf. end of Sect. 4.4). Such are the properties of mixture like y, T (see (4.94), (4.236), (4.240), (4.225)) but also the chemical potentials ga. Note that also heat flux is transformed as (4.118) (with functions (4.223)) and therefore heat flux is mixture invariant in a non-diffusing mixture (all = o) in accord with its measurability. But heat flux is mixture non-invariant in a diffusing mixture, consistently with our expectation of difficulties in surface exchange (of masses) of different constituents with different velocities together with heat. We note that all formulations of heat flux used in linear irreversible thermodynamics [1, 120] (cf. Rems. 11 in this chapter, 14 in Chap. 2) are contained (by arbitrariness of rjp) in expression (4.118) for heat flux in a diffusing mixture. 

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