Phase change, heat transfer condensation

Ihe values of the film coefficients of heat transfer h, and accordingly those of the overall coefficient U, vary by orders of magnitudes, depending on the fluid properties and on whether they undergo phase change, that is, condensation or boiling. Thus, the correct estimation of U is very important in the design of heat transfer equipment. 

Latent heat transfer Transfer of heat required to bring about a phase change (e.g., condensation or vaporization) in a fluid. (Compare to sensible heat transfer.) Many heat exchangers involve both latent and sensible heat transfer. 

Heat transfer is a particular promoter of the development of local chemistries that are very different from the bulk fluid chemistry, in particular, where phase changes occur. Early condensates from acid gas/vapor streams can be very concentrated and corrosive relative to bulk condensates as in the cases of carbonic acid from steam, sulfuric acid from flue gases, and hydrochloric acid from refinery overhead streams. Initially, benign condensates or cooling fluids can concentrate due to intermittent contact with surfaces hot enough to promote concentration or dryout, as in the... 

Carey van P (1992) Liquid-vapor phase-change phenomena. An introduction to the thermophysics of vaporization and condensation processes in heat transfer equipment. Hemisphere, New York Celata GP, Cumo M, Mariani A (1997) Experimental evaluation of the onset of subcooled flow boiling at high liquid velocity and subcoohng. Int J Heat Mass Transfer 40 2979-2885 Celata GP, Cumo M, Mariani A (1993) Burnout in highly subcooled water flow boiling in small diameter tubes. Int J Heat Mass Transfer 36 1269-1285 Chen JC (1966) Correlation for boiling heat transfer to saturated fluids in convective flow. Ind Eng Chem Process Des Develop 5 322-329... 

Carey VP (1992) Liquid-vapour phase-change phenomena. Hemisphere, Washington, DC Collier SP (1981) Convective boiling and condensation. McGraw-Hill, New York Ha JM, Peterson GP (1998) Capillary performance of evaporation flow in micro grooves an analytical approach for very small tilt angles. ASME J Heat Transfer 120 452 57 Hetsroni G, Yarin LP, Pogrebnyak E (2004) Onset of flow instability in a heated capillary tube. Int J Multiphase Flow 30 1424-1449... 

Carey, Liquid-Vapor Phase-Change Phenomena An Introduction to the Thermophysics of Vaporization and Condensation Processes in Heat Transfer Equipment... 

In pharmaceutical systems, both heat and mass transfer are involved whenever a phase change occurs. Lyophilization (freeze-drying) depends on the solid-vapor phase transition of water induced by the addition of thermal energy to a frozen sample in a controlled manner. Lyophilization is described in detail in Chapter 16. Similarly, the adsorption of water vapor by pharmaceutical solids liberates the heat of condensation, as discussed in Chapter 17. 

If the no-phase-change restriction does not rigorously apply, a simple design procedure can be formulated based on the results discussed in Section III, where it is shown that thermal equilibrium is quickly achieved in gas-liquid systems because of the large heat effects associated with evaporation or condensation. Although the total mass transfer between the phases may be small, it is not unrealistic to assume that the gas and liquid phases have the same temperature at each axial position. 

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. 

At present, waste heat exhausted from the ICE is removed with any efficient radiator system through direct apparent heat exchanging. On the contrary, organic chemical hydrides can recuperate the chemical energy of endothermic reaction heat during exhausted heat removal. Heat transfers accompanying the phase change of evaporation and condensation of aromatic products and unconverted reactants will certainly facilitate the removal of heat from the ICE parts, with adoption of any new radiator system compelled. 

A Carnot engine with a steady flow rate of 1 kg/sec uses water as the working fluid. Water changes phase from saturated liquid to saturated vapor as heat is added from a heat source at 300° C. Heat rejection takes place at a pressure of lOkPa. Determine (1) the quality at the exit of the turbine, (2) the quality at the inlet of the pump, (3) the heat transfer added in the boiler, (4) the power required for the pump, (5) the power produced by the turbine, (6) the heat transfer rejected in the condenser, and (7) the cycle efficiency. 

A schematic representation of the combustion wave structure of a typical energetic material is shown in Fig. 3.9 and the heat transfer process as a function of the burning distance and temperature is shown in Fig. 3.10. In zone I (solid-phase zone or condensed-phase zone), no chemical reactions occur and the temperature increases from the initial temperature (Tq) to the decomposition temperature (T ). In zone II (condensed-phase reaction zone), in which there is a phase change from solid to liquid and/or to gas and reactive gaseous species are formed in endothermic or exothermic reactions, the temperature increases from T to the burning surface temperature (Tf In zone III (gas-phase reaction zone), in which exothermic gas-phase reactions occur, the temperature increases rapidly from Tj to the flame temperature (Tg). 

The transfer of heat is one of the most basic and best-understood unit operations. Heat can be transferred between the same phases (liquid-liquid, gas-gas) or phase change can occur on either the process side (in the case of condensers, evaporators, and reboilers) or the utility side (in the case of steam heater) of the heat exchanger. 

As discussed previously, in many propulsion systems the recovery of a large fraction of the dissociation energy in the nozzle expansion through recombination is difficult to achieve. While the assumption of frozen flow with respect to recombination reactions appears necessary for many heat transfer rocket nozzle expansions, it is possible that condensation phenomena are sufficiently rapid to provide near equilibrium flow with respect to phase changes. For this special possibility, phase equilibrium in the presence of frozen dissociation, it has been shown theoretically (48) that the performance in terms of specific impulse of propellants containing light metallic elements can exceed the performance of hydrogen. 

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