Heat transfer without phase change

Heat transfer in two-phase flow can be studied most conveniently by adopting a two-part classification heat transfer without phase change, and heat transfer with phase change. In this section, the non-phase-change problems will be discussed the phase-change problem will be dealt with in Section III. 

Two-phase mass transfer and heat transfer without phase change are analogous, and the results of mass-transfer studies can be used to help clarify the heat-transfer problems. Cichy et al. (C5) have formulated basic design equations for isothermal gas-liquid tubular reactors. The authors arranged the common visually defined flow patterns into five basic flow regimes, each 

Stratified, smooth interface, laminar liquid, laminar liquid. 

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The two goals of this chapter were to provide a critical review of the current state of the art in the field of two-phase flow with heat transfer and to provide procedures which can be used for the design of tubular fluid-fluid systems. Both heat transfer without phase change and with phase change were discussed in detail. In each case the analysis was based on an understanding of the flow patterns and the hydrodynamics of the system. 

Temperature gradients are the driving force for the heat transfer, where heat is transported from a region with high temperature to a region with low temperature. Heat transfer can occur within a phase or between phases. According to Fourier s law, the one-dimensional conductive heat transfer without phase change can be described by... 

The representations described above are for sinple situations commonly encountered. However, more conplex cases exist. If the heat capacity is not constant, then the line for heat transfer without phase change is a curve. For phase changes involving multiconponent systems, because the bubble and dew points are at different tenperatures, the line representing the phase change is not horizontal. For partial condensers and vaporizers, the representation on a T-Q diagram is a curve rather than a straight line. 

It was shown that in heat transfer with phase change it is necessary to understand the phase-change phenomenon on the molecular level to model effectively the mass- and heat-transfer processes. An analytical expression for the rates of vaporization and condensation was developed. It was also shown that the assumption of a saturated vapor phase greatly simplified the calculation without a significant loss in accuracy for given examples. However, experimental verification of this simplified assumption is currently lacking. 

This is equipment where heat transfer through a solid wall without chemical reactions takes place. The heat transfer between two fluids without phase change is considered as an example. 

The values of the film coefficient for liquids without phase change are usually larger than those for gases, by one or two orders of magnitude. Nonetheless, the liquid-side heat transfer resistance may be the major resistance in an equipment heated by saturated steam. Film coefficient for liquids without phase change can be predicted by correlations such as those in Equations 5.8a, 5.12a, or 5.13. 

Weisberg et al. [54] are among other researchers who all provided additional information and considerable evidence that the behavior of fluid flow and heat transfer in microchannels or microtubes without phase change is substantially different from that which occurs in large channels and/or tubes. 

In two-phase flow, the boiling temperature falls in the direction of flow as a result of the pressure drop. This results in a change in the driving temperature drop decisive for heat transfer along the flow path. Calculation of the heat transfer without simultaneous investigation of the pressure drop is therefore impossible. The fundamentals steps for this shall be explained in the following. 

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