Bhand, S C

Gaur, M S

Gogate, D V

English

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IdEFFECT  OF IONIC CU RRENTS ON H EAT-TRAN SFERB. C. BHAND, M. S. GAUH a n d  D. V. GOGATEi ’HYStCS DeI'AIITMMNT, MaDUAV COLLliOE, VxKUAM UNIVERSITY, l^JJAIN {liev&ived Augubt 22, H)H2)ABSTRACT. Elcct-rolytic (lonio) c u n m ls  iug ]jioduo(i(l ui a wvuk ('iGcIiolyto KivinR nsH to tliG frnnorHtiou ol Inibblns and thGir (iffoct on boat IniUHtn ih utiulipd Tliiu jiIhUjuUii wiidh wm> uHGd ns hunting aurfacos luid the eiilianeoinenl id heat lian sle j under conlrolli'd bubble j^eueration was deteriiuned for dillorent valuea of tlie exeesH of tenijK'mture A0 ol the hoaimjr Hiirl'aeo nvej the surj’oundinn liquid ( 'hnnges in the value of the heat t ransfei’ coe^ eien t 7i’ resuhing 1mm variation in the aieu of the heating surfaee and in tlie execMH of leni|jei'ature are exhibited grn|jhieally and the |M>ak valui' ol 7?' obtained under the above conditiouH is then deter minedJ N T JU)1J UU T J  ONHeat transfer between a heating solitl surfaee and a boiling liquid has been the. subjeid of many investigations din ing recent years (Nukiyama 1934; Drew andMuellei, 1937; Addoms, 1948; Bromley, J9.'5(), Uao, Desai and Gogato, 1960). Tn the process of heat transfei in boiling litpiids, tlie inei’ease of temperature of the heating surface causes an increase in the rate of bubble evolution and also in the number of bubble nucleation-sitos. These tAvo effects can be simulated by electrolytic generation of bubbles and there is the added advantage that it can be controlled independently of the heating surface tomporatine. It thus offers possibilities of a better- understanding of the mechanism by Avhich heat is trans­ferred In our exper-imenis Imhbles are produced electrolytically on a heated platinum wire immersed in a dilute electrolyte.Mixon and du Pont (1959) have investigated the influeiiee of electrolytic bubble evolution on heat-transfer and have concluded that there is a slight but iriconclusivo evidence that an increase in the ionic current lowers the attainable heat flux m the high current region.in a preliminary note Edkie, Rao and Gogate (1961) have briefly described the changes in the heat transfer coefficient 7d resulting from the production of ionic currents under a constant difference of temp. M  between a heated platinum wire and the surrounding liquid. It  appears from their results that there is a sudden fall in the value of ‘h' immediately after the peak value is reached. Butthey do not seem to have obtained any definite and reliable observations for the sudden fall in with increase in the strength of the ionic currents.The work reported in this paper was undertaken to examine heat transfer enhancement resulting from the controlled bubble generation by means of ionic185186 S. C. Bkand, M. S. Gaur and D. V. Gogatecurrents and to investigate the lowering of heat flux in the high ionic current region.E X P E K I M E N T A f.A thin idatinum wire (ahoiit 0 1 m.ni. in dianictor and 10 onus, in length) was stretclied horizontally along the axis of a hollow metal cylinder open at both ends and the system (wire and the metal cylinder) was kept immei’sed m a through containing a. very dilute eleedrolyto, say water, containing a few drops of dilute sulphuric acid (Fig. 1). A potential difference ranging from 10 to 100 volts|t’lg. 1. (a) \V\V, plaLijiuiii wire, (b) Cy , cylinder, (C) r , i '— standaid  low rcBiHiancob(using a dry battery) was established between the wire and metal cylinder giving rise to a ladial-clcctru! field The ionic (eleidrolytic) cuiTcnts generated in this way, were measured by means of milli-ammeterMeasurements of the current flowing through the wire and its lesistance arc used for dotcTininiiig both the power input and the tempefature of the wire. Under equilibrium conditions, the heat input to the wire becomes identical to the lieat transfer from the wire to the liquid.For the accurate determination of the current flovung through the platinum wire the latter was made to form one arm of a low resistance bridge (Kelvin’s Double Bridge) as shown in Fig. 1. Tn the other arm (balancing .arm), two standard low rosistBiices r, r' (each fraction of an ohm) are connected in series with a small variable I'esistance. One of the two low resistances (r) is shunted by a key so that it ('.an be introduced into the circuit when desired. The poten­tial drop across the second low' resistance (r') is measured by means of a poten­tiometer and from this the aiscurate value of the current passing through the platinum wire can bo easily doterinined.Effect of Ionic Currents on Heat-Transfer 187To start the oxpormunit a. vory small fiimait of the orclci of lOm. a. (wlmsh does not heat the wjie appreciably) is jiassetl through Hu» platinum wire, which is kept in one, arm of the low resiat-aiice bridge referred to above and a balance is obtained by adjusting the resistances in the other arm of th(* bridge. The additional standard low resistance (r) is now introdmied, and this disturbs the balaniie pre- viously obtained The ciinent in the pliitinum wire is now gradually increased to leatorc the balance Tlie change in teuiperatiu-e A/9, can be calculated by means of the relation — {rjR^a), where r, the additional standard low rests, tance introduced in balancing arm, R^ ,^ the resistance of the platinum wire at ()°C and a, the tonij). coefft. of resistance for the wire, are known.If now some potential difference is established between the iilatinum wire, and the metal cylinder, electrolysis starts in the licjuid and ionic, currents begin to How between the wire and the cylinder Bubbles are found to rise from diffei'ent points on the wiic flue to evolution of gas in the pi-ocess of'electrolysis and the temp, of the wire is slightly lowered. The wire is now brought back to its initial tem­perature (timperature before the comiueiicemont ol ionic ciin-ents) by passing a little more current through it from ihe battery and the heat exchange between it and the surrounding liquid is then measured. Ihe heat flux ijjA {A --  sui'bice area, 2nrl of the uu're) lor unit difference of temperature betwoon the wire and the surrounding liquid is kiiou ii as the heat transfer cooHicieiit 7i.’. We have investigated the variation ol 7d with increasing values of ^0 for different strengths of loiiii^  cuiTents and with jilatinum wires of different diametm-s The diffeient values of ^0 could be obtained by choosing suitable values of the additional standard low resistance 7 ’ in the balancing arm. A typical set of observations showing the variation of the heat transfer coe.flicient 7i.’ with the strength of tlie ionic euiTimt ‘i.’ for a fixed value of AO is given in Table 1.TABLE IWire diameter =  O'lO lu.m., AO — 6'3"C, =  0.0873 cal./sq. cni./Hec/°CSI.No'i mimDi-aiiipLofr f hCrtl /hc] cm./ "C/HCli.iC K )I 20 1 30 0 1107 1 2082 40 l.OO 0 .1 2 7 4 1.4583 00 1 78 0 1348 1 5454 80 1 00 0 .1 3 0 8 1 507/) 100 2 .0 0 0 1351 I 5476 120 2 ,0 8 0 1310 ] 5017 140 2 14 0 1148 1 3148 1(10 2 20 0 1029 1 179{) 180 2 20 0 0983 ] 12010 200 2 .3 0 0 0909 1.04111 220 2 .3 4 0 0802 0 .9 8 812 240 2 38 0 .0726 0.83113 200 2 41 0 0015 0 .7 0 5188 S. C. Bliand, M. S. Gaur and D. V. GogateK E S I T L T H  A N D  D I S C U S S I O NTn Hiudying ih« variation of ‘A’ with ionic cmTent, the difference of temp. ^0 between the wire and the surrounding liquid was kept constant by keeping a definite value of V’, in the balancing arm The values of {hjh^)*, where is the heat transfer coefficient in the absence of ionic currents, were then plottoii against Jog i, where is the strength ot ionic current in miUiamperes. One such plot is shown in h^ ig. 2(cui'V0 A ), where the value of AO was maintained constant at 6.3°C.TAholog 'i —>F ig . 2. 'Pho plot o f (hjho) against log i  being ionic current in  miHLamporeH) using platinum  wim of diaiiUitor 0.10 nini., a t diffei-eut values o f Afi (A) (B ) 10.0"0, (C) 15.2“0 ,(D) 20.J°C .This process was repeated for the values of AO =  10.0“C 15.2°C, and 20,1'’C and the resulting graphs B, 0 , D  are shown in Fig. 2. It  is obvious from these graphs that there is a definite increase in the peak value of with increasing values of AO. For instance, at the value of AO =  6.3°0, max”^  is equal to 1.567whereas with A6 =  20.1®0, (A/^o)max*" comes out to be 2.559.* I t  iH found more convonieni to  uae tho ra tio  {hjho) than  ‘A’ alone, while considering the variation  o f heat transfer w itli increase in  the ionic currents becauso, though the values o f  7t’ m ay vary for diffeiunt sets of observations under different conditions of tem perature, pressure, hum idity e tc ., tho value of {hjha) rem ains sufficiently cousiatent and um fonn.The effect of varying ili(‘ fhainoior of tlic platimmi wire on tlie raft? of ]ieat transfer wiis then Ktiidied For this piirpost'. wires of four different, diameters were used Tlie variation ol 'h' with ionic, (invents for Ihcso waives of different diameters IS depicted m Fig. :h The different curves of Fig ih clearly indicateEjfect of Ionic Currents on Heat-T'ransfer 189lof? y,—>Kig :i Thd plot o f {hlho) against log t luiuntmuiug AO -  b .Td ainl u.sing platinum wivos of dilfcn^nt diaiiu'i.nrs, (A) OlOuim., ( li)  0.19 nini , (C!) 0.31 m m ., (])) 0 37 mm.that the maximum rate of heat transfer {li^ Y^ ,atni) goes on tailing slowly as the diaiiu^ -ter of the wire increases though m this ease the change in the peak value ol 7/,' is not very great, even when the diameter of the wire is increased to four times its original value.The variation of the peak value of 7t’ with (i) increase in the value of HO and (ii) increase m the diameter of the platinum wire is shown in Table IT.TABLE 11MITVo.Wire dmmoli* uiV -  0 10 mm M  =- 6.3^(!(/j.//io)max«* wire dmiiujtcr in min. {hjh1 0 3 1 567 0 10 1.5072. 10 0 1 938 0 J9 J .4333! JO 2 2 222 0.31 1 3234. 20 I 2 659 0 37 1 226190 S. C. Bhand, M. S. Oaur and D. V. GogateOur results indieate very clearly that for every value of there is a dofiiiito enhanecnient of heat transfei up to a (jertain value of ionite current and this is followed by a reffular decrease in heat transfei" in the higher ionic current region.Moreover the graphs of Fig. 2 and 3 can bo taken as a conclusive evidence to show that though 'h' becomes maximum for an oi>timum value of i, it goes on falling gradually and not suddenly as reported by Edkie, Rao and Gogate (1961) with increasing strengths of ionic currents.In calculating AO wo cannot suppose that the temp, of the platinum wire alone is raised up to a certain point by the current passing through it and that the temperature of the surrounding liquid remains constant, though the rise in tern- peraturo of the liquid is made negligibly small by employing a large (juaiitity of the liquid and very thin platinum wires. A correction for this very small change in AO was however applied by plotting change in AO against time and the values of AO corrected in this way were used for determining the value of "h’tn the above experiments the platinum wire was always kept at a positive potential with respect to the cylmdei" We have (heckod up that the nature of the phenomena as shown by the graphs of b^ g. 2 ami 3 remains unaltered by rovors- nig the polarity, though it causes a sliglit change in the value of ‘h\C () N C L  V  S I O N  S(a) There is an optimum value of ‘i ’ (the strength of the loniii current) for which the heat transfer coefficient 'h' becomes maximum.(b) The value of ‘A’ goes on falling gradually after attaining the peak value with increase in the strength of ioniii currents,(c) The enhancement of heat transfer becomes greater when the difference of temperature AO between the heating surface and the surrounding liquid is increased.(d) For a given value of AO, the rate of heat transfer goes on falling when the area of the heating surface is enlarged.A C K  N O W  L  K D O M 10 N T  SWe wish to express our deep sense of gratitude to Principal Dr. IS. M. Singh ‘Suman’ for giving us all necessary facilities to carry on this investigation in the Madhav College. We aie also grateful to our Vice-Chancellor Dr. G. L. Datta for his constant encouragement in this work.R  13 V E 3^  E  N C E  SAddnms, .T N. 1948, citod in  ‘Heat-Transm iBBion’ by  W illiam  H . Me Adams (MoGmw- H j I I  Rook Co. In c  , Now York). 1964, 1182.Brondov, L. A. I960, Chem Ettg. Proqr., 46, 221.llrew , T . B . and Muellor, A. C., 1937. Tram . Am . Inst. Chem. Engrs., 33, 449.E dkio, n .  G ., R a o , R . R . and G ogate, D. V .. I9H1. J  Sri. dt Industr. Mrs., 2 0 B , 548.M ixon, F . O. and du P o n t, E . I . ,  1959, Chem Eng. Progr., 55, 49N ukiyaiiia, S ., 1934, J .  Sor. Mech. Engrs. [Japan), 37, 367, 853 -5 4 .R ao , R . D ., D esai, H , 8 , and G ogate, D . V ., 1900, Ind. J .  Ph^s,, 84, 456.

Effect of Ionic Currents on Heat-Transfer

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Effect of Ionic Currents on Heat-Transfer

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