Desai, H S

Gogate, D V

Rao, R D

English

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A t^OTE ON HEAT TRANSFER AND FILM BOILINGK. JD. RAO, H. K. DKSAl am b I), V. GOGATEPhysics D kpaktmknt, M. S. Univeksity , Bahoda {R e c e h fed  M a y  31, I960)ABSTRACT. Heat iransfor botweon (ilectrically heated metal wir’es and difl'erent boilmg liquids has boon studied and chnraotonstir boiling curves are obtained by jilotting the heat flux q j A  against the exeess of temperature of the wire over the boiling point. The 1 elation botweon the heat transfer coeffiriont h  and lias also boon studied. The values of maximum heat flux and critical tomporature differonoe are calculated fo  ^ the different wires and liquids used in this investigation.1 N  T 1< 0  D U 0 T T O NIt irt a matter of liomnion experience that when a red hot metal ib (jiieiuslied in water, the metal lirst cools slowly, tlion rapidly and then slowly again. This can be taken as a good illustration of the throe possible typos of boiling, viz.— film boiling, nucleate boiling and natural convection boiling. Film boiling is that type of boiling which occurs when a vapour film exists between a heated surface and a boiling liquid. Tn nucleate boiling, vapour bubbles originate fi om different parts of tin heated surface. Natural convection boiling takes place when the difference o( temperature between the heated surface and the liqnid is small. In the opera I ions of jets and rockets, there are frequent contacts between a boiling liquid and a 1 ot surface and this is the condition for film boiling. Tn an electri­cally heated bi, iler or an atomic power plant where the heat input is the controlled variable, there is always a danger that the temperature of the heated object, may rise abruptly if the heat input is near the critical heat flux ( q j A ) .  This danger becomes much more pronounced if the value of the heat input is above the maximum heat flux (q lA ) m a x ‘ If abrupt temperature rise is sufficiently large, it may give rise to sudden expansion and weakening of certain parts of the system, someimies causing breakage.In view of the above importance of film boiling and nucleate boiling, we have tried (1) to investigate the effect of quemihing electrically heated wires in different liquids and (2) to study the heat transfer, bylneans of characteristic boiling curves (incorporating free convection boiling, nucleate boiling and film boiling ) between cylindrical metal wires and boiling liquids.Drew and Mueller (1937) and others have studied heat transfer to boiling liquids by steam condensing method. Nukiyaraa (1934) succeeded in obtaining al­most complete boiling curves by electrically heating thin platinum wires submerged50456A Note on Heat Transfer and Film Boiling 457in boiling water. Natural convection boiling and nucleate bailing o f  water for different pressures have been studied b y  Addom s (1948) using thin plathium wires. Extensive study o f  film boiling was math; by Brom ley (1950) using various organic liquids.During our study o f  heat transfer, Ave have obtained characteristic boiling curves for a num ber o f  liquids and the results o f  our (‘xperiments oji heat transfer betw een cylindrical m etal wires and boiling liquids have been described in this note. The com plete boiling curves [plots o f  log (g/A) against log A/J for the vaious liquids used have been obtained for tliffcirent wires, and the maximum heat flux and the critical temperature difference havt  ^ been calciilatcil. The heat flux (g /^j4) for  unit difference o f  temperature between tlui u^ ire and tlie surroundingliquid is know n as the heat transfer ooelficient l i . Thus h  —- ^ r e l a t i o nbetween this coefficient 1i and the temperatuit! difference has also been sutdied for liquids such as water, carbon tetrachloride, turpentine, etc. and some typical results have been graphically illusti ated (Fig. 2 ).E X P E R I M E N T A LThe experim ental arrangement consisted of a simple W heatstone bridge Avith ratio arms o f  1000 ohms each. A  thin platinum wire which was submerged in the boiling liqu id  Avas included in the third arm of the bridge in seiies with an amm eter, while a small rheostat and a Eureka wii-e bridge vdth a sliding con tact form ed the fourth  arm o f  the bridge.The platinum  wire as as alloAvccJ to remain in the boiling liquid for some time before passing a current through it so that it attained the temperature o f  the boiling liquid. The resistance o f  the wire could then be calculated at this tem ­perature, i f  i?o> the resistance of the wire at 0 ‘’ C and a, the tiuiiperalurc coefficient o f  resistance fo r  the wire are known. A  very small cuirrent wliich docs not heat the Avire appreciably, was then passed through the wire and a balance was obtained by  adjusting the rheostat and hy sliding the contact on the Eureka wire bridge. The current through the wire was then increased so as to  raise its temperature. This increases the resistance o f  the wire, thus disturbing the balance previously obtained. The con tact is now shifted so as to restore the balance. Kuowm g the value o f  this shift and also the resistance per unit length of the Eureka a w o , the change in resistance Ai^ o f  the platinum Avire could be calculate . .excess o f  tem perature o f  the wire above the boding point o f  the liquid could thenAJ2be determ ined b y  means o f  the relation AtR E S U L T S  A N D  D I S C U S S I O N458 U. 2). U(M>, H. 8, Deaai and D. V, OogateQZJfThe input power q  is equal to where C  is the current passing thrgugh the wire, R  is its resistance and J  is the mechanical equivalent of heat, The heatCurve I—^Platinuiu-water. Curve II— ^Platmum-carbontotrajihloride,Figi 2. Curves showing the variation of h  with the escooss of temperature^ Cmwe I—^Pltttinum-water. Curve II—Fiatinum-carbontretraohloride.flux is given, by g(A where A is the surface area 27trl of the wire of length I and radius r. Typical curves showing the variation of qlA wiih the excess of tempera­ture M as also the relation between the heat transfer coefficient h I J )\ AAt/and for water and carbon-tetrachloride are exhibited in T’igs. (1) and (2) respectively.The values of the maximum heat flux and critical temperaturedifference ^tcnt for the different wires and liquids used as also the values of the slopes calculated for natural convection boiling and nucleate boiling together with the heat transfer coefficient (/^ ) at the peak value arc shown in Table I.A Note on Heat Transfer and Film Boiling 469TABLE rLiquidSlopes for( z ) ent(^ '■Imoa:cal/cm.2800°CNat.conv.boilingNucl.boiling1 m a x  cal/sec. cm3Water Platinum 0  91 3 26 15.8 r r e 0.800(B.P. lOO^C) Turpontino -d o - 1 2 3.6 12.03 39.8°C 0 302(B.P. 106°C) V =  0.005 cm. Copper 1 . 0 4.60 JO.90 3G..3°C 0.302j- =  0.0026 cm. Tungsten 0 .9 1 6 1 2 . 6 43.0^0 0.275NaphthaleneV =  0.0028 cm. Platinum 0.77 2 . 8 11.48 39 0.288(B.P. 316°C,Cop)jor 0.83 2.76 10.72 39.8"C 0.203Tungsten 1 . 0 1.7 12.59 0.331Oarbontetraohloride Platinum 0 . 6 ^.57 0 .9 19.96°0 0 355(B.P. 7 T C )Tungsten 0.71 3.0 8.7 2 2  9"C 0 355AVhen the heat flux exceeds {qlA),nax system passes from nucleateboiling regime to film boiling regime after passing through a transient state of unstable film boiling. This unstable (transient) state is shown by the dotted curve in Figs. (1) and (2). In the state of stable film boiling a vapour film is formed between the wire and the liquid. This film acts as a barrier in which the heat flow is due more to conduction than to convection. The formation of tliis barrier (vapour blanket) naturally diminishes the heat flow from the wire to the liquid and hence the value of heat transfer coefficient h is also decreased as indicated by the graphs in Fig. 2. These graphs also mdicate that if the heat flux is still further increased, the value of h goes on decreasing further,n o N O L U S I G N S(a) The material and dimensions of the wire do not seem to affect the valueof the maximum heat flux (q/A)^ax niuch as the properties of the liquid, especiallythe latent heat of vaporisation.(b) The continued decrease in the value of h with the increase of heat flux in the film boiling regime seems to be due to a slight increase in the thickness of the vapour film.R E F E R E N C E SAddoms J. N ., 1948,“ Hoat Transmission” by William H  McAdams, MoGraw-Hill Third Edition 1954 page 382 m Asian studonts’ Edition.Brornloy, L. A ., 1950, C h e m . E n g .  P r o g r . , 46, 221. 'Drew, T. B. and Miiollor, A. C,, 1937, T r a n s . A m .  I n s t .  C h e m . K n g r s .,  33, 449. \Niikiyama, S., 1934, J S o e .  M e c h .  K n g r s .,  {.T a 'pan), 37, 307., S53-54.460 R. Z). Rao, ZT, S. Desai and D. V. Gogate

A Note on Heat Transfer and Film Boiling

http://arxiv.iacs.res.in:8080/jspui/bitstream/10821/2272/1/A%20note%20on%20heat%20transfer%20and%20film%20boiling_R%20D%20Rao.pdf

A Note on Heat Transfer and Film Boiling

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