International Journal of Latest Research in Science and Technology Volume 4, Issue 2: Page No , March-April 2015

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1 International Journal of Latest Research in Science and Technology Volume 4, Issue 2: Page No , March-April ISSN (Online): EXPERIMENTAL STUDY FOR COMPARISON OF OVERALL HEAT TRANSFER CO-EFFICIENT IN HELICAL COILS OF FIXED LENGTH USING WATER S.S. Pawar, Pritam Shetty, Ancila John, Sairaj Pawar, Pravin Jagtap Department of Mechanical Engineering, SIES Graduate School of Technology, Navi Mumbai, India Nomenclature: ä coil curvature ratio a inner radius of tube (m) Cp specific heat at constant pressure (J/kg K) d i inner diameter of coil (m) d o outer diameter of coil (m) D mean helix diameter of coil (m) U o overall heat transfer coefficient hi inside convective heat transfer coefficient (W/m 2 K) ho outside convective heat transfer coefficient (W/m 2 K) k thermal conductivity of water (W/mK) Lc length of coil (m) ṁ mass flow rate of cold water (kg/s) De Dean number, Re (d/2r) 1/2 N number of coil turns Ni Inner Nusselt number, hidi/k Q heat transferred to cold water (W) R mean helical radius of the coil (m) ñ density of water in kg/m 3 v water velocity inside the coil (m/s) Re Reynolds number, ñvd/ì Abstract In the present study the effect of surface area and centrifugal force on the overall heat transfer coefficient of the helical coil was investigated experimentally. Two helical coils of curvature ratio and were tested for laminar and turbulent flow regimes under constant vessel temperature conditions. Both the coils were of 5m in length and had different number of turns. Experiments were implemented for water in isothermal condition. The variation in the parameters like overall heat transfer coefficient, nusselt number, inner and outer heat transfer coefficient of both the coils were then compared under the same experimental conditions. Keywords Helical coil, Heat transfer, Overall heat transfer, Curvature ratio, Reynolds number I. INTRODUCTION Helical coil heat exchangers are of great use in industrial applications such as power generation, nuclear industry, process plants, heat recovery systems, refrigeration, food industry, etc due to its compact structure and high heat transfer coefficient. Helical coils of circular cross section have been used in wide variety of applications due to simplicity in manufacturing. Flow in curved tube is different from the flow in straight tube because of the presence of the centrifugal forces. These centrifugal forces generate a secondary flow, normal to the primary direction of flow with circulatory effects that increases both the friction factor and rate of heat transfer. The intensity of secondary flow developed in the tube is the function of tube diameter (d) and coil diameter (D) as presented by Dravid et al [1]. Due to enhanced heat transfer in helical coiled configuration the study of flow and heat transfer characteristics in the curved tube is of prime importance. The determination of the relative advantage of using a helically coiled heat exchanger versus a straight tube heat exchanger for heating liquids was provided by D.G. Prabhanjan [2]. A review of flow and heat transfer 161

2 characteristics in curved tubes is provided by Naphon and Wongwises [3]. Parametric analysis of helical coil heat exchanger with various correlations given by different researchers for specific conditions was carried out by Pramod, Mandar and Rajkumar [4]. Also recently, a critical review of heat transfer through helical coils of circular cross section is presented by S.S. Pawar et al. [5]. Heat transfer coefficient in helical coil heat exchanger under turbulent flow condition was presented by Coronel and Sandeep [6]. From the literature survey it is noticed that there is no evidence of heat transfer coefficient in helical coils for same length and different coil turns. This is most significant to understand what is the actual contribution to increase or decrease overall heat transfer coefficient by centrifugal force induced in the coil or by surface area of the coil. II. OBJECTIVE The main objective of this experimental study was the determination of overall heat transfer coefficient in helical coils for same length having different coil turns. The actual effect of centrifugal force and change in surface area of the coil is very essential to check experimentally by keeping same surface area for different coil turns. This is very important while deigning industrial helical coil heat exchangers. Hence, present experimental work is undertaken to understand the effect of these two parameters on the overall heat transfer coefficient for fixed length having different coil turns. III. EXPERIMENTAL SETUP A schematic diagram of the experimental setup is as shown in the fig. (1). The physical dimensions of helical coils used in the present investigations are given in Table 1. A hot water vessel of size 450 mm diameter, 650mm height and 4mm thickness of mild steel material of construction was used to house the coils. The water in the vessel outside the coil is in a natural convection heat transfer mode and does not have any stratified flow pattern. Hence, the effect of stratification in the water vessel is ignored. Two electrical heaters were used in hot water vessel of 5KW and 3KW fixed at the bottom of the vessel. During the isothermal process one of the heaters was switched on and off as and when required to maintain a constant hot water vessel temperature of 55 ± 0.5 C. Two alcohol thermometers having a range of -10 to 110 C were mounted at the inlet and outlet of the coil as shown in the fig. 1. A centrifugal pump of 1HP was used to pass the test fluids through the coils and a rotameter was used to measure the flow rate of the water flowing through the coil. Figure 2: Experimental setup in 2D Figure 1: Experimental setup in 2D ISSN;

3 Figure 2: Experimental setup TABLE 1. PHYSICAL DIMENSIONS OF HELICAL COILS USED Helical coils ä L(mm) Do(mm) di(mm) do(mm) N Coil Coil ISSN:

4 IV. EXPERIMENTAL PROCEDURE The reservoir and the vessel were filled with water after which the water from the reservoir tank was pumped through the coil by using 1HP centrifugal pump. The flow rate of the water was measured by using a rotameter. Flow rate for each reading was verified by noting down the time taken by the water from the outlet of the coil to fill a measuring cylinder of 1 liter to reduce the error in the flow measurements. As the water passed through the coil it was heated by the water in the vessel equipped with two heaters. Inlet and outlet temperature of the water as well as the vessel temperature were noted down only after the system was allowed to come to a steady state condition. A temperature of 55 C was maintained to carry out the experiments in isothermal conditions. As soon as the required temperature was attained, the first reading was taken and the remaining readings were taken at a time interval of 15 minutes. V. METHODOLOGY Thermo physical properties of the water inside the helical coil were evaluated at an average bulk temperature. Heat loss from the hot water in the vessel to the cold water passing through the coil is give as: temperature and the inner Nusselt number simultaneously. A first approximation was made for the average wall temperature and was used to calculate the bulk temperature. All properties for the Reynolds number and the Prandtl number were evaluated at the bulk temperature and the Nusselt number was calculated using Eq. (4). The inside heat transfer coefficient, hi, was then determined from The wall temperature, Tw, was then determined from, T bulk and Ai are the average bulk temperature (average of the inlet and outlet temperatures) of the processing fluid and the inside surface area of the coil, respectively. The average bulk temperature was based on the average of the inlet and outlet temperatures, which were both bulk temperatures. The outside heat transfer coefficient was then calculated from the equation as : By using the above equation overall heat transfer coefficient can be calculated as: Where Ao is the outside surface area of the coil, Tv is the vessel temperature and Tw is the wall temperature as calculated by using equation (6) Where is the overall surface area of the coil and is the temperatue difference between the water in the vessel and the bulk temperature of the water in the coil. As per D.G Prabhanjan [7] the overall heat transfer coefficient can be described in terms of thermal resistances for a cylindrical tube as: The radii for the inner wall and the outer wall of the tube are ri and ro, respectively, and the thermal conductivity of the coil is k. It was necessary to determine the inside heat transfer coefficient, hi, to proceed with the calculations for ho. The relationship of Rogers and Mayhew [8] based on the bulk temperature was used to determine the inner Nusselt number, Nui, of the coil, given as: VI. RESULTS AND DISCUSSION The results of the experiments carried out for different flow rates are summarized as follows: All the important parameters such as overall heat transfer co-efficient, inner heat transfer co-efficient, outer heat transfer co-efficient and nusselt s number were compared against the dean number. The overall heat transfer co-efficient in the helical coil heat exchanger increased with the flow rate and approached maximum value at higher flow rates in both the coils as shown in fig. 3. Also, it is seen that the overall heat transfer is more in the coil of curvature ratio than the coil with curvature ratio of Deviation in the values of Uo for both the coils was not found to be large as there is a difference of just one turn. Average values of Uo for both the coils are shown in the table 2 for same experimental conditions. It is clearly seen that Uo is greater in the 220mm coil. Since the wall temperature is not known, the film temperature used to evaluate the Reynolds number, Re, and the Prandtl number, Pr, was also unknown. Therefore an iterative approach was used to determine the wall ISSN:

5 We also observed that as the curvature ratio of the coil was increased, the outer heat transfer co-efficient in the coil with larger curvature ratio also increased as seen in the fig. 6 Figure 3: Plot showing Uo vs De It was also observed that hi was increasing as the flow rate was increased in both the coils. The following graph (fig. 4) shows that hi is more in the coil of diameter 220mm than in coil of diameter 300mm. Figure 6: Plot showing ho vs De The average values are shown in the table below: TABLE 2: AVERAGE VALUES OF DIFFERENT PARAMETERS FOR EACH COIL Sr. Coil 1 Coil 2 % no. Parameters (220mm) (300mm) deviation 1 De Nu Uo Figure 4: Plot showing hi vs De A plot of Nu vs De is shown in the graph (fig. 5) below. It is clearly understood from the graph that undoubtedly, the helical coil with curvature ratio has higher heat transfer rate than the coil with curvature ratio hi ho VII. CONCLUSION In the present study the effect of surface area and centrifugal force on the helical coils was investigated. From the experiments conducted in the present work it was seen that the increase of overall heat transfer coefficient in the helical coil was solely due to the effect of centrifugal force and did not depend on the surface area of the coil. Heat transfer coefficient was more in the helical coil with smaller helix diameter. As the helix diameter decreases there is an increase in the overall heat transfer co-efficient due to the increase in the centrifugal force and development of secondary flow. However further studies are need to be done to determine the heat transfer enhancement in helical coils of non circular cross sections and also for non Newtonian fluids. Figure 5: Plot showing Nu vs De ISSN:

6 REFERENCES 1. Dravid, A. N., Smith., K. A., Merrill, E.A., and Brian, P.L.T., (1971) Effect of secondary fluid motion on laminar flow heat transfer in helically coiled tubes, AIChE Journal, Vol. 17(5): D. G. Prabhanjan, G. S. V. Ragbavan and T. J. Kennic, lnt. Comm. Hcnr Mas.s Tnm& Vol. 29. No.2. pp , (2002) 3. Paisarn Naphon, Somchai Wongwises, A Review Of Flow And Heat Transfer Characteristics In Curved Tubes, Fluid Mechanics, Thermal Engineering and Multiphase Flow Research Lab 17 September Pramod S. Purandare, Mandar M. Lele, Rajkumar Gupta, Parametric Analysis of Helical Coil Heat Exchanger, International Journal of Engineering Research & Technology (IJERT) (2012) 5. S.S Pawar, et al. A critical review of heat transfer through helical coils of circular cross-section 6. Heat Transfer Coefficient in Helical Heat Exchangers under Turbulent Flow Conditions, Pablo Coronel and K.P. Sandeep 7. Devanahalli G. Prabhanjan, Timothy J. Rennie, G.S. Vijaya Raghavan, Natural Convection heat transfer from helical coiled tubes, International Journal of Thermal Sciences 43 (2004) G.F.C Rogers, Y.R Mayhew, heat transfer and pressure loss in helically coiled tubes with turbulent flow, International journal of heat and mass transfer 7 (1964) ISSN: