The CRC Handbook of Mechanical Engineering. Chapter 4. Heat and Mass Transfer (776127), страница 19
Текст из файла (страница 19)
Additional information on features of the boiling curve when the liquid poorlywets the surface or when L/Lb is small can be found in Carey (1992). To make this discussion concrete,we will assume that the ambient liquid surrounding the immersed body is at the saturation temperaturefor the ambient pressure. If the surface temperature of the immersed body is controlled and slowly© 1999 by CRC Press LLCHeat and Mass Transfer4-85FIGURE 4.4.3 Pool boiling regimes for an independently controlled surface temperature.increased, the boiling curve will look similar to that shown in Figure 4.4.3.
The axes in this plot arelogarithmic scales. The regimes of pool boiling encountered for an upward-facing horizontal flat surfaceas its temperature is increased are also indicated in Figure 4.4.3. The lateral extent of the surface ispresumed to be much larger than Lb. At very low wall superheat levels, no nucleation sites may be activeand heat may be transferred from the surface to the ambient liquid by natural convection alone and q²increases slowly with Tw – Tsat.Eventually, the superheat becomes large enough to initiate nucleation at some of the cavities on thesurface.
This onset of nucleate boiling (ONB) condition occurs at point c in Figure 4.4.3. Once nucleateboiling is initiated, any further increase in wall temperature causes the system operating point to moveupward along section d-f of the curve in Figure 4.4.3. This portion of the curve corresponds to thenucleate boiling regime. The active sites are few and widely separated at low wall superheat levels.
Thisrange of conditions, corresponding to segment d-e of the curve, is sometimes referred to as the isolatedbubble regime.With increasing surface superheat, more sites become active, and the bubble frequency at each sitegenerally increases. Eventually, the active sites are spaced so closely that bubbles from adjacent sitesmerge together during the final stages of growth and release.
Vapor is being produced so rapidly thatbubbles merging together form columns of vapor slugs that rise upward in the liquid pool toward itsfree surface. This higher range of wall superheat, corresponding to segment e-f of the boiling curve inFigure 4.4.3, is referred to as the regime of slugs and columns.Increasing the wall superheat and heat flux within the regime of slugs and columns produces anincrease in the flow rate of vapor away from the surface. Eventually, the resulting vapor drag on theliquid moving toward the surface becomes so severe that liquid is unable to reach the surface fast enoughto keep the surface completely wetted with liquid.
Vapor patches accumulate at some locations andevaporation of the liquid between the surface and some of these patches dries out portions of the surface.If the surface temperature is held constant and uniform, dry portions of the surface covered with avapor film will locally transfer a much lower heat flux than wetted portions of the surface where nucleateboiling is occurring.
Because of the reduction in heat flux from intermittently dry portions of the surface,the mean overall heat flux from the surface is reduced. Thus, increasing the wall temperature within the© 1999 by CRC Press LLC4-86Section 4slugs and columns region ultimately results in a peaking and rollover of the heat flux. The peak valueof heat flux is called the critical heat flux (CHF), designated as point f in Figure 4.4.3If the wall temperature is increased beyond the critical heat flux condition, a regime is encounteredin which the mean overall heat flux decreases as the wall superheat increases. This regime, which isusually referred to as the transition boiling regime, corresponds to segment f-g on the boiling curveshown in Figure 4.4.3.
The transition boiling regime is typically characterized by rapid and severefluctuations in the local surface heat flux and/or temperature values (depending on the imposed boundarycondition). These fluctuations occur because the dry regions are generally unstable, existing momentarilyat a given location before collapsing and allowing the surface to be rewetted.The vapor film generated during transition boiling can be sustained for longer intervals at higher walltemperatures. Because the intermittent insulating effect of the vapor blanketing is maintained longer,the time-averaged contributions of the blanketed locations to the overall mean heat flux are reduced.The mean heat flux from the surface thus decreases as the wall superheat is increased in the transitionregime.
As this trend continues, eventually a point is reached at which the surface is hot enough tosustain a stable vapor film on the surface for an indefinite period of time. The entire surface then becomesblanketed with a vapor film, thus making the transition to the film boiling regime.
This transition occursat point g in Figure 4.4.3.Within the film boiling regime, the heat flux monotonically increases as the superheat increases. Thistrend is a consequence of the increased conduction and/or convection transport due to the increaseddriving temperature difference across the vapor film. Radiative transport across the vapor layer may alsobecome important at higher wall temperatures.Once a surface is heated to a superheat level in the film boiling regime, if the surface temperature isslowly decreased, in general the system will progress through each of the regimes described above inreverse order. However, the path of the boiling curve may differ significantly from that observed forincreasing wall superheat, depending on whether the surface heat flux or temperature is controlled.Experimental evidence summarized by Witte and Lienhard (1982) implies that the path of the transitionboiling curve is determined, to a large degree, by the wetting characteristics of the liquid on the solidsurface.
For a given wall superheat level in the transition boiling regime, a higher heat flux is generallyobtained if the liquid wets the surface than if it poorly wets the surface. For systems that exhibit contactangle hysteresis, the transition boiling curves obtained for decreasing and increasing wall superheat maytherefore be somewhat different. The transition boiling curve for decreasing wall superheat may besignificantly below that for increasing superheat for such circumstances, as indicated in Figure 4.4.4.For an electrically heated surface, the rise in temperature associated with the jump from nucleate tofilm boiling at the critical heat flux is very often large enough to melt component materials and burnout the component.
As a result, the critical heat flux is often referred to as the burnout heat flux toacknowledge the potentially damaging effects of applying this heat flux level to components cooled bynucleate boiling. Once the jump to film boiling has been made, any further increase in applied heat fluxincreases the wall superheat, and the system follows basically the same film boiling curve as in thetemperature-controlled case.Correlations of nucleate pool boiling heat transfer data have typically been used as tools to predictnucleate boiling heat transfer in engineering systems and heat exchangers. Many investigators haveproposed methods of correlating data of this type; so many, in fact, that a complete discussion of themall could easily fill a major portion of this section.
In this section, three of the more commonly usedcorrelation methods will be mentioned. However, before proceeding, two aspects of the interpretationof such correlations are worth noting. First, experimental data indicate that the subcooling of the liquidpool has a negligible effect on the nucleate boiling heat transfer rate. Consequently, the pool boilingcorrelations are generally regarded as being valid for both subcooled and saturated nucleate boiling.Second, it has also been observed that at moderate to high heat flux levels, a pool boiling heat transfercorrelation developed for one heated surface geometry in one specific orientation often works reasonablywell for other geometries and/or other orientations.
Hence, although a correlation was developed for a© 1999 by CRC Press LLC4-87Heat and Mass TransferFIGURE 4.4.4 Relative locations of the nucleate transition and film transition portions of the pool boiling curve.specific geometry and orientation, it may often be used for other geometries, at least at moderate to highheat flux levels.Having taken note of the above points, a commonly used correlation for nucleate boiling heat transferdeveloped by Rohsenow (1962) isq ¢¢m l h fgéùsêúêë g(rl - r v ) úû12æ 1 ö=ç÷è Csf ø1r-s rlPr[] ùúé c T - T (P )satlê pl wh fgêëúû1r(4.4.4)Values of r = 0.33 and s = 1.7 are recommended for this correlation, but for water s should be changedto 1.0.
The values of Csf in this correlation vary with the type of solid surface and the type of fluid inthe system. This empirically accounts for material property and/or wetting angle effects. Recommendedvalues of Csf for specific liquid–solid combinations are given by Rohsenow (1962), but whenever possible,an experiment should be conducted to determine the appropriate value of Csf for the particular solid–liquidcombination of interest. If this is not possible, a value of Csf = 0.013 is recommended as a firstapproximation.As noted previously, the pool boiling curve generally exhibits a maximum heat flux or CHF at thetransition between nucleate and transition boiling.
This peak value is the maximum level of heat fluxfrom the surface which the system can provide in a nonfilm-boiling mode at a given pressure. Themechanism responsible for the CHF has been the subject of considerable investigation and debate overthe past five decades. As the heat flux increases, bubbles generated at the surface coalesce to form vaporcolumns or jets. Perhaps the most widely cited CHF model postulates that the CHF condition occurswhen Helmholtz instability of the large vapor jets leaving the surface distorts the jets, blocking liquidflow to portions of the heated surface.
Continued vaporization of liquid at locations on the surface whichare starved of replacement liquid than leads to formation of a vapor blanket over part or all of the surface.¢¢ isAccording to Zuber (1959) for a flat horizontal surface, the predicted maximum heat flux qmax© 1999 by CRC Press LLC4-88Section 4é s g (rl - r v ) ùqmax¢¢ = 0.131rv h fg êúr2vêëúû14(4.4.5)but Lienhard and Dhir (1973) recommend that the constant 0.131 in the above relation be replaced with0.141.
![](https://s.studizba.com/z.php?f=/templates/si/i/noavatar.png&w=100&h=100&t=1"){kind=avatar}
