Analysis of Natural Convention Heat Transfer Enhancement in Finned Tube Heat Exchangers

Transcription

1 Invention Journal of Research Technology in Engineering & Management (IJRTEM) ISSN: ǁ Volume 1 ǁ Issue 7 ǁ Analysis of Natural Convention Heat Transfer Enhancement in Finned Tube Heat Exchangers Mahendra J. Sable Professor, Mechanical Engineering Department, Government College of Engineering, Jalgaon, , India ABSTRACT: Most of the engineering problems require high performance heat transfer components with progressively less weight, volumes, accommodating shapes and costs. Air cooled heat exchangers are subjected to air on outer side of heat exchanger surface on in heat recovery systems like economizers gases are subjected on one side of tube surface. On air or gas side heat transfer coefficient is less. Extended surface (fins) are one of the next exchanging devices that are employed extensively to increase heat transfer rates from tubular heat exchangers. The rate of heat transfer depends on the surface area of fin available for exchanging the heat transfer rate from the primary surface of cylindrical shape. Present study focuses on enhancement of heat transfer by using both circular and elliptical type of fins. The present paper attempts to examine trend of heat transfer coefficient experimentally and by using CFD software for various types of elliptical fins with i) varying elliptical ratio, ii) changing orientation of mounting of heat exchanger tube with elliptical fins, iii) varying spacing or fin density. KEY WORDS: Natural convection, Heat transfer enhancements, Elliptical fin, Fin orientation, Fin density. INTRODUCTION The majority of passive techniques employ special surface geometry or fluid additives for enhancement of heat transfer without direct application of external power. Whenever it is difficult to increase the rate of heat transfer either by increasing heat transfer coefficient or by increasing the temperature difference between the surfaces and surrounding fluid, the fins are commonly used. Fin and tube heat exchangers are widely used to transfer heat between liquid and gas. Examples include condensers and evaporators in HVAC & R applications (heating, ventilation, air conditioning and refrigeration, water and oil cooling in vehicles or process plants etc. The dominant heat transfer resistance (up to 85% of the total thermal resistance) in these process is located on the gas side due to the lower thermal conductivity of the gas. In order to improve the heat transfer rate the exterior surface area is increased by adding fins. The disadvantage of adding fins is that the gas side frictional pressure drop increases. N. Nagarani et al.[3] has analyzed the heat transfer rate and efficiency for circular and elliptical annular fins for different environmental conditions.. Elliptical fin efficiency is more than circular fin. If space restriction is there along one particular direction while the perpendicular direction is relatively unrestricted elliptical fins could be a good choice. He stated the advantages of elliptical fin as (i)in cross flow, the aerodynamically shaped tubes will reduce the pressure drop of the external fluid to a great extent in the sub critical range. S. R. MCILWAIN [4] in his study compared heat transfer around a single serrated finned tube and a plain finned tube. He stated that with comparable bundle geometry, serrated fin finned tube heat exchangers transfer more heat than plain finned ones. H. Huisseune and C. T Joen [5], in there study experimentally determined the heat transfer and friction correlation of a single row heat exchanger with helically finned tubes. The transversal tube pitch was parametrically varied. Gulay Yakar and Rasim Karabacak [6], studied the effects of holes placed on perforated finned heat exchangers at different angles on the Nusselt and Reynolds numbers. A. Nuntaphan, T. Kiatsiriroat, C.C. Wang [7], in their experimental work, studied heat transfer and friction characteristics of crimped spiral finned heat exchangers with dehumidification using forced convection. Their study experimentally examines the air-side performance of a total of 10 cross flow heat exchangers having crimped spiral configurations under the dehumidification. The effect of tube diameter, fin spacing, fin height, transverse tube pitch, and tube arrangements are examined. Rene Hofmann, Friedrich Frasz, Karl Ponweiser [8],in their study compared heat transfer and pressure drop in forced convection of different shaped fins on tube heat exchangers. Three different finned-tube shapes were investigated. The I-shaped helical, U-shaped helical and I-shaped serrated fins. They found that the heat transfer capacity of U-shaped finned-tube is better than that of an I-shaped finned-tube. The comparison of the serrated and solid finned tubes shows, the dimensionless heat transfer coefficient for serrated finned tube rows in staggered arrangement is greater than that for the solid tube bundle. Gregory J. Zdaniuk, Louay M. Chamra, Pedro J. Mago [9],in their experimental work determined the performance of heat exchanger with inside helical fins at different helix angles between 25 and 48 and at different fin height to diameter ratios between and M.J.Sable, S.K.Bhor[10], performed computational analysis for enhancement of natural convection heat transfer on vertical heated plate by multiple V-fin array. They used CFD analysis because it eliminates the cost associated with experimental setups for different geometries of fins and varying variables such as fin height, thickness, fin spacing which saves time and cost of experimentation. Thus in the context of above work, it was decided to determine heat transfer performance of elliptical fins for varying elliptical ratios and for changing orientations and fin spacing s, both experimentally and using CFD analysis which has not been attempted so far. Volume 1 Issue 7 6

2 EXPERIMENTAL INVESTIGATION Figure - 1 show the tube with elliptical fins. The experimental set up is as shown in the Figure - 2. The annular circular and elliptical fin made of steel is vertically mounted on the circular tube. The various dimensions are taken from reference(3).the circular fins have the outer diameter of 99mm,thickness 1mm and the space between the fin is 8mm. The horizontal circular tube is placed on two supports which is 98 mm above the experimental table to avoid ground effect. Length, diameter and thickness on the horizontal circular pipe made of steel are 100mm,40mm and 3mm. Electrical heating coil with 0.5 kw capacity is kept inside the tube. Size of the box is 500x300x300mm. The process is done with free convection. The temperature of air entering test specimen is measured as the ambient temperature and test fin temperatures are measured using K type thermocouples at the base and fin tip. The temperatures of the circular tube at inlet and outlet are measured using a thermocouple. For elliptical fin the major and minor axis ratio are 2.0 & 2.5 with same circular tube dimension. Surface area of both fin are equal. The readings are taken for circular and elliptical fins with two types of elliptical ratios. For elliptical finned tube readings are taken for two orientations i.e. major axis facing vertical and horizontal.. Fig1: Elliptical Fin Tube Fig.2: Experimental setup The following formulas are used used to calculate heat transfer coefficient, Actual heat input : Q = IV Surface area for circular fin tube : A = 2π n(r 2 o - r 2 i ) + πdl Surface area for elliptical fin tube : A = 2π n(ab- r 2 i ) + πdl Heat transfer coefficient : h = Q/ A. ( T s T atm ) COMPUTATIONAL ANALYSIS Finite numbers of fins with thickness1mm are assumed and length of 100mm is assumed. A domain has to be built around the fin to study mass flow and thus the heat flow from the fin array, because the area of interest is the outside of fin array, which is the interface between the air and fin surface. Thus, the domain consists of air. Domain size is selected ensuring that the temperature changes at domain boundaries are negligible and the conditions can be taken as atmospheric conditions. This simplifies the case to a great extent. Figure 3 shows the computational domain selected for the purpose of analysis. As per post processing outcome, the assumed domain size is justified. Fig. 3 Solid Model from Pro-e Fig. 4 Meshing of Air domain in ICEM-CFD Volume 1 Issue 7 7

3 Fig. 5 Meshing of Solid domain Fig. 6 Prism Mesh - Fluid domain in ICEM-CFD Fig. 7 Boundary conditions definition in CFX - pre Fig. 8 Temperature Contour on Elliptical Fin surface The geometric model created in Pro-E is as shown in Figure 3 and exported to ANSYS ICEM-CFD. The meshing scheme used for meshing the geometry is tetrahedral mesh. As trend of heat transfer co-efficient is to be examined, prism mesh is used for interface on fluid side. Mesh size of solid domain is same as prism mesh size of fluid domain. Air domain is assumed inside tube which is the place for heat input whose mesh size is same as solid domain. The air, solid and fluid domain mesh is as shown in Figure -4, Figure-5 and Figure -6 respectively. The mesh file was then imported into ANSYS CFX-preprocessing for defining boundary conditions as shown in Figure -7. Computational analysis involves the application of heat transfer and fluid dynamics principles. It involves three steps i.e. modeling, preprocessing and post processing. The results of CFX postprocessing are as shown in Figure - 8 to Figure 10. Fig. 9 Temp. contour for circular fin Fig.10 Velocity Vector-CFX-post Fig 11 velocity streamline RESULTS AND DISCUSSION The surface area of fins is maintained same as that of circular fin for all types of elliptical fins. Also total surface area of fins for all types of fin density i.e. fin spacing is maintained same by changing outer diameter of fin. The graphs of heat transfer coefficient is plotted for all types of elliptical fins for both experimental and CFD analysis. For first case major axis of elliptical fins is kept vertical and in second case is kept horizontal. In another study by using CFD analysis effect of fin spacing or fin pitch is determined by changing fin density of circular fins over same length of tube. The results are as shown in following figures. Volume 1 Issue 7 8

4 Heat transfer coeff. W/m 2 K Heat transfer coeff.(h) W/m 2 K Heat transfer coeff. W/m 2 k 12 (8.0) 16 (5.3) 18 (4.7) 19 (4.5) 20 (4.2) 21 (4.0) 22 (3.5) 28 (2.7) Temperature K Temperature K Temperature K heat transfer coeff. Analysis of Natural Convention Heat Transfer Enhancement in Finned Tube Heat Exchangers CFD Results: W/m Ver. Hor Fin Ratio ( a/b ) Fig. 12: Variation of HTC Vs Ratio of major to minor axis (a/b) of Elliptical fin Elliptical ratio (a/b) Fig.13: Variation of Tube surface temp.(t s ) & Fin Surface temp.(t F ) Vs Elliptical ratio (a/b) for Vertical orientation. TS TF Heat transfer coeff.(h) Vs Fin Density(spacing) Series Elliptical ratio (a/b) Fig.14: Variation of Tube surface temp.(t s ) & Fin Surface temp.(t F ) Vs Elliptical ratio (a/b) for Horizontal orientation TS TF TS TF. Fin Density ( spacing) Fin Density ( Fin Spacing) Fig 15. Variation of HTC Vs Fin Density i.e. Fin spacing for Circular fins. Fig.16: Variation of Tube surface temp.(t s ) & Fin surface Temp.( T F ) Vs Fin Density for Circular fins Experimental Results: Heat Input ( W) a/b=1 a/b=2 a/b= Heat input(w) a/b=1 a/b=2 a/b=2.5 Volume 1 Issue 7 9

5 Fig. 17: Variation of HTC Vs Fin ratio (a/b) of Elliptical fins in Vertical orientation Fig. 18: Variation of HTC Vs Fin ratio (a/b) of Elliptical fins in Horizontal orientation From Figure-9, it is seen that, for circular fin the isotherms are radically symmetrical, fin surface area temperature gradually reduces only along it s radial direction so heat transfer is equal in all directions. From Figure-8, it is observed that, for elliptical fins the radial geometry of temperature field disappears slightly away from the tube wall. It is observed from temperature contours of all types of elliptical fins oriented in any direction and for circular fins, the temperature field is symmetric about vertical axis of fins and about horizontal axis it is not symmetric. This is because in natural convection the flow regime is upwards, that is why it is seen that the surface temperature in upper part of fins is more than the lower one. CFD analysis for elliptical fins with different elliptical ratios (Figure-12) is carried out. Temperature difference plays a vital role in heat transfer rate. As shown in Figure-12, it is observed that elliptical fins have somewhat higher heat transfer coefficient (HTC) than circular one for fins of small elliptical ratio of major axis to minor axis(a/b) only when major axis is horizontally oriented. This is because as we know that the boundary layer thickness in case of vertical plate, is increasing upwards and when minor axis is vertical, the height of vertical plate is less and therefore the largest thickness of boundary layer developed is less and therefore there will be no or less interference of boundary layers developed along adjacent fins. But when we go for higher elliptical ratios (a/b=3) as shown in fig.12, the heat transfer coefficient again decreases. This is because, the effective area along minor axis is reduced as minor axis radius is very small, though boundary layer interference is very less as compared to small a/b ratio fins. When major axis of elliptical fins is vertically oriented, boundary layer thickness developed will be larger, which results in reduction in heat transfer rate. From Figure-13 and Figure-14, it is observed that the temperature difference between surface temperature of tube (T S ) and average fin surface temperature (T F ) goes on increasing as elliptical ratio (a/b) of fin increases. As temperature difference between tube surface and fin surface increases, heat transfer rate decreases,(figure-12 to 14). Figure-15 and Figure-16 shows the effect of fin density i.e. fin spacing on heat transfer rate. It is observed from Figure-15 that heat transfer coefficient is higher when fin density is near 20 fins over the same tube length of 110mm. i.e. when fin spacing is in the range of 4mm to 4.5mm. when fin density is less than 20 i.e. spacing higher than this range, effective area available for heat transfer is less resulting in reduction in heat transfer. When fin density is increased more than 20 fins i.e. fin spacing is reduced than the above observed range, though effective area available for heat transfer is more but there might be more interference of boundary layers developed which results in reduction in heat transfer rate. Experimental results as shown in Figure-18, also explains that elliptical fins having small elliptical ratio(a/b) when mounted as major axis orientating horizontally, gives better heat transfer rate even somewhat better than circular one. But when major axis is oriented vertically, it gives poor heat transfer rate for all elliptical ratio fins as shown in Figure-17. CONCLUSION 1. It is observed both by experimental and CFD analysis that for elliptical fins having ratio of major axis to minor axis in the range of 1.5 to 2 and when major axis of fin is oriented parallel to horizontal, gives about 5-6% greater Heat Transfer Coefficient ( HTC) than the circular one of same surface area. 2. It is clear from Figure -13 and Figure -14 that as ratio of major axis to minor axis of elliptical fin increases, the difference between tube surface temperature and average temperature of fin surface increases which results in reduction in HTC. 3. HTC decreases with increase in heat flux, i.e. HTC is inversely proportional to heat flux. This puts an upper limit on heat flux.fig.17 and fig HTC is greater when fin density is near about 20 i.e. fin spacing is near about in the range of 4-4.5mm. For reduction in fin spacing than the above range, HTC goes on decreasing.fig HTC is greater with fin spacing in optimum range i.e. in the range of 4-4.5mm with small difference in tube surface temperature and average fin surface temperature and with less interference of adjacent boundary layers. Fig.15 and fig.16. NOMENCLATURE: h = Heat transfer coefficient, A = Total convective surface area, r 0 = Outer radius of circular fin, r i = Outer radius of circular fin, a = Semi-major diameter of elliptical fin, b = Semi-minor diameter of elliptical fin, d = Outer diameter of tube = 2 r i Volume 1 Issue

6 L = Length of tube. REFERENCES [1] Incropera F. P., DeWitt D. P., 1996, Fundamentals of heat and mass transfer, 4 th Edition, John Wiley & Sons Publication [2] Yunus A. Çengel, 2004, Heat Transfer- A Practical Approach, SI units 2 nd Edition, TMG Publication [3] N. Nagarani, 2010, Experimental Heat transfer Analysis on Annular Circular and Elliptical Fins.. International Journal of Engg. Science and Tech., Vol 2(7), pp [4] S.R. Mcilwain, Feb. 2010, A Comparison of Heat Transfer around a single Serrated Finned Tube and a Plain Finned Tube, UJRRAS 2(2) [5] H. Huisseune, C.T. Joen, 2010, Thermal Hydraulic Study of a Single Row Heat Exchanger with Helically Finned Tibe, ASME, Vol 132. [6] Gulay Yakar and Rasim Karabacak, Jan, 2010, Effect of Holes placed on Perforated Finned Heat Exchangers at Diff Angles on the Nusselt and Reynolds numbers, Scientific Research and Essay, Vol, 5920 PP [7] A. Nuntaphan, T. Kiatsiriroat, C.C. Wang, Heat Transfer and Friction Characterstics of Crimped Spiral Finned Heat Exchangers with Dehumidification. [8] Rene Hofmann, Friedrich Frasz, Karl Ponweiser, 2008, Heat Transfer and Pressure Drop Performance Comparision of Finned Tube Bundles in Forced Convection, Wseas. Transactions on Heat and Mass Transfer, Issue 4, Vol. 2, PP [9] Gregory J Zdaniuk, Louay M. Chamra, Pedro J. Mago, 2008, Experimental Determination of Hear Transfer and friction in Helically-Finned Tubes. Experimental Thermal and Fluid Science 32, PP [10] M.J.Sable, S.K.Bhor,2011, Computational Analysis For Enhancement Of Natural Convection Heat Transfer On Vertical Heated Plate By Multiple V-Fin Array IJAER Volume 6, Number 13 (2011) pp Volume 1 Issue