Evaporation critical heat flux

Figure 2.42 shows boiling curves obtained in an annular channel with length 24 mm and different gap size (Bond numbers). The heat flux q is plotted versus the wall excess temperature AT = 7w — 7s (the natural convection data are not shown). The horizontal arrows indicate the critical heat flux. In these experiments we did not observe any signs of hysteresis. The wall excess temperature was reduced as the Bond number (gap size) decreased. One can see that the bubbles grew in the narrow channel, and the liquid layer between the wall and the base of the bubble was enlarged. It facilitates evaporation and increases latent heat transfer. 

Katto Y (1978) A generalized correlation for critical heat flux for the forced convection boihng in vertical uniformly heated round tubes. Int J Heat Mass Transfer 21 1527-1542 Khrustalev D, Faghri A (1996) Fluid flow effect in evaporation from liquid-vapor meniscus. ASME J Heat Mass Transfer 118 725-747... 

Lazarek, G.M., and Blake, S.H., (1982), Evaporative heat transfer, pressure drop and critical heat flux in small vertical tube with R-113, International Journal Heat and Mass Transfer 25, 7, pp 945-960. 

Even when the critical heat flux is reached not all the liquid is vaporized. Therefore an upper limit for the critical heat flux is obtained, which is necessary for complete evaporation of the liquid. Once again this is found from an energy balance... 

Lazarek, G. M., and Black, S. H., Evaporative Heat Transfer, Pressure Drop and Critical Heat Flux in a Small Vertical Tube with R-113, Int. J. Heat and Mass Transfer, 25(7), 945—960 (1982). 

Number of researchers reports the liquid forced convection inside narrow channels is a most valuable form of heat removal from heat sinks [13-16]. However, the respective key issues are the maximum attainable heat flux by using liquid forced convection, and its value in comparison with the preeminent alternatives of boiling critical heat fluxes. We believe that maximum heat flux could be accomplished by using the inverted meniscus principle of evaporation coupled with excluding of vapor... 

The extension of these PECs to two-phase heat transfer is complicated by the dependence of the local heat transfer coefficient on the local temperature difference and/or quality. Heat transfer and pressure drop have been considered in the evaluation of internally finned tubes for refrigerant evaporators [14] and for internally finned tubes, helically ribbed tubes, and spirally fluted tubes for refrigerant condensers [15]. Pumping power has been incorporated into the evaluation of inserts used to elevate subcooled boiling critical heat flux (CHF) [16, 17]. A discussion of the application of enhancement to two-phase systems is given by Webb [373],... 

For higher flow velocities, the minimum in the critical heat flux was much less pronounced. When the quality region is entered, there is a complex interaction between the flow patterns existing within the channel and the heat transfer behavior. A particularly important case is that of evaporation in horizontal evaporator tubes as used, for instance, in many refrigeration and air-conditioning plants. Here, due to the action of gravity, the liquid phase tends to be... 

FIGURE 15.124 Comparison of critical heat flux data for tubes with cosine variation of axial heat flux and uniform heat flux evaporation of water at 6.89 MPa in a 12.7-mm bore tube (from Keeys et al. [301], reproduced by permission of AEA Technology pic). 

Macrolayer evaporation models. In this class of models, the critical heat flux is considered to be governed by a macrolayer underneath a vapor clot as illustrated in Fig. 15.120. This type of model is analogous to those suggested for pool boiling, and it has been pursued by a number of authors including Katto [101], Mudawwar et al. [316], and Lee and Mudawwar [317]. 

Prediction of Critical Heat Flux in Annular Flow. In annular flow, the situation to be modeled is illustrated in Fig. 15.106. There is a thin liquid film on the channel wall that has a flow rate T per unit periphery. Droplets are being entrained from this film into the vapor core at a rate mE (mass rate of entrainment per unit peripheral area, kg/m2s) and are being redeposited from the core at a rate mD (kg/m2s). In addition, the liquid film is being evaporated at a rate q"/i,g per unit peripheral area. Thus, the rate of change of T with distance is given by... 

FIGURE 15.134 Fraction of liquid evaporated before onset of critical heat flux as a function of mass velocity for a plain tube and for a tube with helical tape inserts q" is the flux corresponding to the evaporation of all the injected water (from Moeck et al. [326], with permission). 

When evaporation is present, two-phase heat transfer should be taken into consideration. A number of correlations have been derived to predict the critical heat flux condition as a function of temperature difference between the fluid and the heated surface. A comparison between the predicted and experimental heat flux values is shown in Figs. 5.6-5.8 for hydrogen, nitrogen, and oxygen, respectively. The evaporation usually occurs on the shell side. 

Inasmuch as the critical AT corresponds to a maximum heat flux, this AT will permit a maximum duty for an evaporator, reboiler, or other boiling equipment. In practice, industrial equipment is designed to operate at slightly less than the critical A T. This gives a performance which is somewhat less than the optimum, but it provides insurance against exceeding the critical AT. 

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