HEAT TRANSFER AND PRESSURE DROP CHARACTERISTICS IN A DOUBLE-PIPE HEAT EXCHANGER FITTED WITH A TURBULATOR

Transcription

1 HEAT TRANSFER AND PRESSURE DROP CHARACTERISTICS IN A DOUBLE-PIPE HEAT EXCHANGER FITTED WITH A TURBULATOR Warakorm Nerdno Putchaya Somravysn Smth Eamsa-ard Faculty of Engneerng Mahanakorn Unversty of Technology Emal: smth@mut.ac.th Pongjet Promvonge Faculty of Engneerng Kng Mongkut s Insttute of Technology Ladkranbang ABSTRACT The purpose of ths study s to nvestgate heat transfer and pressure drop characterstcs n a double ppe heat exchanger ftted wth a helcal-rod nsert. Augmented heat transfer was above the plan tube values. The greatest mprovement of heat transfer was found from helcal-rod nserts where Nusselt numbers ranged from 150% to 160% comparson wth the plan tube values at correspondng Reynolds numbers. It s shown that the pressure drop for the tube wth the helcal-rod nsert s 6 to 9 tmes those of the plan tube for the range of Reynolds numbers tested. Keywords: heat transfer, helcal-rod, turbulence flow, double-ppe heat exchanger 1. INTRODUCTION Heat exchanger s a devce facltatng heat transfer between two or more fluds. It s extensvely used n several ndustres, such as thermal power plants, chemcal processng plants, ar condton equpment, refrgerators, radator for space vehcles as well as automobles etc.[1-2]. Heat exchanger has been classfed n many dfferent ways. The desgn of heat exchanger s complcated, requrng a consderaton of dfferent modes of heat exchanger, pressure drop, szng, long term performance estmaton as well as economc aspect. Most of the desgns are based on compactness of the unt. The compactness may be defned as the rato of the heat transfer surface area on one sde of the heat exchanger to the volume. Generally, a medum heat exchanger wth a surface area densty on any sde greater then 700 m 2 /m 3 s referred to as a compact heat exchanger regardless of ts structural desgn. As a heat exchanger gets older, the resstance to heat exchanger rate ncreases due to foulng or scalng. Ths partcularly true n heat exchangers used n marne as well as chemcal ndustres. Also n some ndustres there s a need to ncrease the heat transfer rate n the exstng heat exchanger. Therefore to mantan the desred heat transfer n exstng heat exchangers several methods have been nvented n the recent years. Heat exchangers have several ndustral and engneerng applcatons. Technques for enhancng heat transfer are relevant to several engneerng applcatons. In recent years, the hgh cost of energy and materal has resulted n an ncreased effort amed at producng more effcent heat exchange equpment. The heat transfer rate can be mproved by ntroducng a dsturbance n the flud flow (breakng the vscous and thermal boundary layers), but n the process pumpng power may ncrease sgnfcantly and ultmately the pumpng cost becomes hgh. Therefore, to acheve a desred heat transfer rate n an exstng heat exchanger at an economc pumpng power, several technques have been proposed n recent years. The great attempt on utlzng dfferent methods s made to ncrease the heat transfer rate through the compulsory force convecton. Meanwhle, t s found that ths way can reduce the szes of the heat exchanger devce and save up the energy. In general, enhancng the heat transfer can be dvded nto 2 groups: One s the passve method, t s the way wthout beng stmulated by the external power such as; surface coatngs (treated surface) may be employed to enhance convecton by promotng turbulence at the surface (specal coatngs are even used to promote condensaton or bolng), rough surfaces are used to amplfy mxng n the boundary layer near the surface ths ntegral roughness s created by reformng and machnng the surface, extended surfaces are commonly employed n heat exchangers to enhance heat transfer by ncreasng surface area, for example, wavy, lanced-offset, and perforated fns, the swrl flow devces for example, a twsted-tape [3-5], a wre-col nserts, tangental njecton devces [6] mpart a tangental velocty component to the flud that ncreases the heat-transfer coeffcent, the convoluted (twsted) tube manfest both extended and roughened surface methods of heat-transfer augmentaton by promotng turbulence [7] and addtves for lqud and gases. The other s the actve method. Ths way requres the extra external power sources, for example; mechancal ads nvolve mechancally strrng the flud or rotatng the heat exchanger, surface-flud vbraton may be employed by ether vbratng the flud or hear exchanger surface, the njecton and the sucton of the flud, the jet mpngement s the stuaton of the flud flow drecton normal to a surface from nozzle njecton or orfce, and

2 the electrostatc felds are used to mx the flud n order to ncrease the heat transfer coeffcent. In ths expermental nvestgaton, a helcal-rod was used as turbulator for enhancng heat transfer rate n the double ppe heat exchangers. All of the experments were carred out at the same nlet condton wth the Reynolds number of the nner tube, Re= Water tank Electrc heater Warm water Hot ar Outer tube Double ppe heat exchanger Cold water Cold ar Inner tube Varac Rotameter Rotameter U-Tube manometer Fan Chller Centrfugal pump Fgure 1: Expermental apparatus. (a) nner test tube (b) helcal-rod Fgure 2: Helcal-rod nserted n the nner tube heat exchanger.

3 2. THEORETICAL ANALYSIS For flud flows n a heat exchanger the heat transfer rate can be expressed as: Q ar = mc & (T T ) (1) pa o The convecton heat transfer from the test secton can be wrtten by: Q conv whereas, = ha(t ~ T ) (2) w b T = (T T ) / 2 (3) b o + and T ~ w = Tw / 15 (4) Where T w presents the local wall temperature and evaluated at the outer wall surface of the nner tube. The averaged wall temperatures are calculated from 15 ponts, lned between the nlet and the ext of the test ppe. The averaged heat transfer coeffcent, h and the mean Nusselt number, Nu are estmated as follows: h = mc & (T T ) / A(T ~ T ) (5) whch p,a o w Nu = hd / k (6) The Reynolds number s gven by Re = UD/ v (7) Frcton factor, f can be wrtten as: P f = L ρ U D 2 2 Where U s mean velocty of the tube. All of thermo-physcal propertes of the ar are determned at the overall bulk ar temperature from Eq. (3). 3. EXPERIMENTAL APPARATUS The arrangement of expermental system of a doubletube heat exchanger was studed and detals of test secton are depcted n Fgures 1 and 2. The double-tube heat exchanger s conssted of two concentrc tubes; the nner tube for the hot ar flow and the outer tube for the water flow. The dameters of the nner and outer tubes were 40 and 65 mm respectvely. The tubes were 2000 mm long and 1 mm thck. Copper and steel tubes were employed for the nner and outer tubes respectvely. The outer tube surface s covered wth nsulaton to prevent heat loss. A helcal-rod was nserted nto the nner tube b (8) for the whole length to create turbulence flow. The 19 mm outer dameter helcal-rod was used wth ptch length of l = 3 mm. Detals of the helcal-rod and test tube geometres are presented n Fgures 2(a-b). Hot ar from a 7.5 kw blower flowed through the nner tube, whle cold water from a water pump enters the annulus. The volumetrc flow rates of the hot ar and cold water were adjusted by the approprate rotameter and two globe valves stuated before the nlet ports. The heatng of the nlet ar was by an electrcal heater and the energy nput was adjusted wth a varable-output voltage transfer. Both the nlet-outlet temperature of the hot ar and the cold water were measured by mult-channel wth ron-constantan thermocouple (type K). It s necessary to measure the temperature at the outer surface of the nner tube altogether 15 ponts for fnd out the average Nusselt number and the whole evaluated temperatures are connected to the date logger sets. Whle the entry and the ext of the nner-outer tubes are set up wth pressure tape for measurng the pressure drop by connectng to the two U-tubes manometer and both are flled wth water In the experments, the counter flow s adjusted by two ball valves to control the drecton of the cold water. For each test run, t s necessary to record the data of the temperature, volumetrc flow rate and pressure drop of the hot ar and the cold water at steady state. Durng the expermentaton, t s consdered to mantan the nlet temperature of the hot ar constantly at 70 o C wth Reynolds numbers n the range of to and the cold water was kept at 25 o C. The varous characterstcs of the flow and the fndng out of the Nusselts number are taken to the consderaton from the Reynolds numbers and the average surface wall temperature. 4. RESULTS AND DISCUSSION The results revealed the heat transfer rate and pressure drop n a double-ppe heat exchanger wth a helcal-rod nsert were presented n Fgures 3 and 4. The expermental results of the heat transfer rates, obtaned n ths study, are shown n Fgure 3. It can be seen that, the helcal-rod nserts gves hgher values of heat transfer rate than those for plan tube, and the means Nusselt numbers ncreased around 150% when compared wth those found from the plan tube. It s defned that a helcal-rod nsert caused turbulence flow and pressure gradent beng created along the radal drecton. The boundary layer along the tube wall would be thnner wth the ncrease of radal velocty and pressure. Therefore, heat could be transferred easly through the flow. Moreover, turbulator would cause flow to be turbulent whch leaded to even better convecton heat transfer. It s depcted that the effect of the helcal-rod nserts decreased at low Reynolds numbers due to the low flow velocty. Thus, the ncrease n Nusselt number was low at smaller Reynolds number, whle t became greater at the hgher Reynolds numbers. From expermental results, the nner tube ftted wth a helcalrod gave the maxmum heat transfer rate at about 160% n comparson wth plan tube.

4 Fgure 3: Relatonshp between Nusselt number and Reynolds number. Fgure 4: Relatonshp between pressure drop and Reynolds number. Pressure drop n the nner tube are shown n Fgure 4 as functon of Reynolds number. The pressure drop of straght flow (plan tube) was also plotted for comparson. It can be seen that the pressure drop were n the smlar trend for both the straght flow and the tube wth a helcal-rod whch the pressure drop of a helcalrod was hgher than that n the plan tube because of the turbulence flow and the dsspaton of dynamc pressure of the flud at hgh vscosty loss near the tube wall. Moreover, the pressure drop had hgh possblty to occur by the nteracton of the pressure forces wth nertal forces n the boundary layer. Therefore, the pressure drop n the nner tube ncreased substantally wth ncreasng Reynolds number. It was observed that the helcal-rod nserts caused turbulence flow nto the

5 tube whch leaded to hgh pressure drop of 6 tmes over the plan tube. 5. CONCLUSIONS Expermental data on the ntermedate range of Reynolds number, Nusselt number and pressure drop, have been presented for case of a double-ppe heat exchanger. From expermental results, t can be observed that the helcal-rod whch placed nsde nner tube results n a pressure gradent beng created n the radal drecton, thus affectng the boundary layer development. The ncreased rate of heat transfer n such flows s a consequence of the reduced boundary layer thckness and ncreased resultant velocty. It s mportant to note that the ncrease n pressure drop wth a helcal-rod generated turbulence flow s hgh than the ncrease n Nusselt number (at the smlar Reynolds number). The maxmum ncrease n Nusselt number s 160% n comparson wth the plan tube, whle the correspondng pressure drop s less than 6 tmes over the plan tube pressure drop. REFERENCES [1] Manglk RM and Bergles AE, Heat transfer and pressure drop correlatons for twsted-tape nserts n sothermal tubes, Part I. Lamnar flows, ASME Journal Heat Transfer, 1993, 115, [2] Marner WJ and Bergles AE, Augmentaton of hghly vscous lamnar heat transfer nsde tubes n constant wall temperature, Expermental Thermmal Flud Scence, 1989, [3] Date AW and Saha SK Numercal predcton of lamnar flow and heat transfer n a tube ftted wth regularly spaced twsted-tape elements, Internatonal Journal Heat Flud Flow, 1990, 11, [4] Hong SW and Bergles AE, Augmentaton of lamnar flow heat transfer n tubes by means of twsted-tape nserts, Internatonal Journal Heat Transfer, 1976, [5] Kumar A and Prasad BN, Investgaton of twsted tape nserted solar water heaters-heat transfer, frcton factor and thermal performance results, Renewable Energy, , [6] Lepna RF and Bergles AE, Heat transfer and pressure drop n tape-generated swrl flow of sngle-phase water, ASME Journal Heat Transfer, 1969, 91, [7] Sam A and Chakroun W, Effect of tube clearance on heat transfer for fully developed turbulent flow n a horzontal sothermal tube, Internatonal Journal Heat and Flud Flow, 1996, 17,