Effect of Louvered Strips on Heat Transfer in a Concentric Pipe Heat Exchanger Somsak Pethkool 1, Smith Eiamsa-ard, Artit Ridluan and Pongjet Promvonge 1,* 1 Department of Mechanical Engineering, Faculty of Engineering, King Mongkut s Institute of Technology Ladkrabang, Bangkok 15, Thailand Department of Mechanical Engineering, Faculty of Engineering, Mahanakorn University of Technology, Bangkok 153, Thailand Abstract: Effects of the louvered strip insertion in a parallel-flow concentric double pipe heat exchanger on heat transfer performance and flow friction are experimentally investigated. The louvered narrow thin metallic strips were placed in the inner tube of the heat exchanger. Used as a turbulator and made of brass, the Louvered strips were.5 mm thick and 9 mm wide. In the system, hot water was introduced through the inner tube, while cold water was flowing through the annulus. The experimentation was performed at the ynolds number 6, to 65, and 8, for the hot and cold water flows, respectively. The inlet hot and cold water temperatures were 55 o C, and o C, respectively. The experimental results showed that the mean Nusselt number were increased up to about 46% in comparison with the plain tube, while approximately 167% of the friction factor was increased, comparative to the plain tubes. Keywords: Heat Transfer Enhancement, Heat Exchanger, Friction Factor, Louvered-strips, Louvered-turbulators 1. INTRODUCTION cently, Energy is essential for domestic industry development and it is the main problem issue not only in Thailand but also around the World. A plenty of energy problems are necessary to be urgently improved and developed. Paying the attention and efficiently economically exploiting energy are the effective way to alleviate energy problems. The heat exchange process has been involved in many engineering applications. Of production process in industrial factories and necessary, is the parameter of great interest temperature. Temperature is one of the most important factors to control production process in each section as well as the quality control of products. Therefore, the degree of temperature at various positions in production processes is significant. In this research, the processes of extracting and transferring maximum heat to intermediate working fluid were focused. Heat exchanger plays the important role in heat exchange processes and is widely used in production processes such as refrigerator and electrical power plants. Accordingly, designing the heat exchange, suitable for each application, is very significant and important and it always has the limitation of heat exchanger size and fluid flow rate. As a result of such restrictions, heat transfer rate is quite low. The enhancement of heat transfer ability to achieve high performance of heat exchangers does reduce the size of the device and the initial investment as well as the area of installation. This paper presented the method of enhancing heat exchanger performance by using the ordinary technique, passive method. The passive method can be applied by installing the turbulent circulation generator In the past, the heat transfer enhancement technique was performed in the widespread application. Those methods were, for instance, the insertion of twisted stripes and tapes [1-3], the insertion of coil wire and helical wire coil [4, 5] and the installation of turbulator [6] in the heat exchangers. The results of those studies had been shown that although heat transfer efficiencies were improved, the frictions of tube were considerably increased. The novel concept to augment heat transfer efficiency using small louvered strips was developed and investigated. The strips were arranged on the brass wire, installed inside the hot water tube. The stripes provided high turbulent and circulation flow, resulting in excellent rate of heat transfer. Figures 1 and present the installation of the louvered strip with the brass core within the copper tube. Three angles of attack of the strip investigated are 17, 6, and 31 degrees in counter clockwise direction with the core. Fig. 1 The installation of the louvered stripes inside the hot water tube Fig. Photograph of the louvered stripes Corresponding author: kppongje@kmitl.ac.th 1
. THEORETICAL ANALYSIS The heat exchange between hot and cold waters was used to obtain the convection heat transfer of the hot water tube. Nusselt Number, Heat transfer rate, and other related parameters of this parallel flow heat exchange were considered as follows; Heat Transfer rate of cold water c c p ( T T ) Q = M C (1) c,outlet c,inlet Heat Transfer rate of hot water h h p ( T T ) Q = M C () M h and M c are mass flow rate of hot and cold waters. C p denotes specific heat coefficient of water. (T c,outlet -T c,inlet ) and (T h,outlet -T h,inlet ) present temperature difference between inlet and outlet cold water and temperature difference of inlet and outlet hot water, respectively. An average heat transfer rate between hot and cold waters, considered in the form of convection heat transfer between copper surface and hot fluid flow, can be written as h,inlet h c Q aver = Q h,outlet Q + Q = Q (3) conv ( ) conv hh Ah cs hf = (4) The average temperature at the copper surface, appeared in equation (5), was obtained from experiment by averaging nine locations of measured temperatures at the tube surface from inlet to exit. = T (5) cs csi /ni The average temperature of hot water can be calculated from equation (6), while convection heat transfer coefficient and Nusselt number (Nu) were defined in (6) and (7) h ( T T )/ = (6) hf h,inlet + h,outlet ( T T )/A ( ) m M hc p h h,inlet h,outlet h cs hf ynolds number and friction factor were given the following relations. = (7) Num = hm D/k (8) h = ρu md/µ (9) f = m P ( ρu / )( L / D) (1) 3. EXPERIMENTAL SET-UP The apparatus and measuring devices were shown in Fig. 3. The temperature at inlet and outlet areas of hot and cold tubes inside the concentric tube heat exchanger was measured using thermocouple, whereas U-tube manometer was used to measure their pressure. Pumps were used to feed hot and cold waters to the cold and hot water tube of double pipe heat exchanger and their flow rates was measured by Rotameter. In this system, the hot and cold waters were kept and their temperatures were controlled within the hot water tank and water chiller. In experimentation, after the hot water attained and controlled at the temperature of 55 o C throughout experimental duration. The hot water was supplied to the heat exchanger. Simultaneously, the cold water, kept within water chiller, was fed to the heat exchanger. The hot and cold waters returned to storing tanks, after heat transfer between them had been operated. During the process, inlet and outlet pressures were measured using U-tube manometer. Surface temperatures of nine locations inside copper tube were measured by Thermocouple type K during of running process. All of data were collected and brought to calculate convection average heat transfer coefficient, Nusselt number and friction factor
Wall Temperature (Tw1...Tw9) Pressure tap Pressure tap Ts6 Ts5 Ts9 Ts8 Ts7 Ts3 Ts4 Ts Ts1 Double pipe heat exchanger Flow meter Bypass Control Cold water tank Water chiller Bypass Flow meter Control Heater Hot water tank Hot water pump Cold water pump Fig. 3 The schematic diagram of experimentation 4. EXPERIMENTAL RESULTS The experimental results were compared with the past correlation of Dittus and Boelter [7] which the correlations were formulated for smooth-surface circular tube or plain tube. Figure 4 shows the plot of Nusselt number and ynolds number from the present work and the past correlation results. The linear behavior of Nusselt number, plotted along ynolds number, was found. The trends of the present work and the past correlation are similar; however, the present work results give the lower value of Nusselt number. Figure 5 depicted the comparison for Nusselt number for the plain tube and the tube fitted with louvered turbulators. In the tube fitted with louvered turbulators, 17, 6, and 31 degrees of inclined louvered stripes were installed. The experimentation showed that 31 degrees of incidence provided the maximum Nusselt number for all ynolds number and had grown linearly. In the case of 17 and 6 degrees, Nusselt numbers were lesser than those of 31 degree, respectively. The lowest Nusselt numbers were found in plain tube over ynolds number range. The reason why the excellent heat transfer was produced by louvered fins is that the fins act as turbulator. The comparison between convection heat transfer coefficients of the plain tube and the tube fitted with louvered turbulator was shown in Figure 6. From experimental consequences, the tube fitted with louvered turbulator with 31 degree of incidence of louvered fins provided the highest convection coefficients for all ynolds number. The relationship between the coefficients and ynolds number of this case is linear. And again the minimum of convection heat transfer coefficients over ynolds number range was found in the plain tube. The tube fitted with louvered turbulators generated the high recirculation turbulent flow, disrupting the growth of the boundary layer, leading to the better heat transfer by slighting the boundary layer thickness. This physics increases convective ability of the heat transfer process from the heat exchanger surface. 3 Experimental 4 Dittus-Boelter 5 3 Nu 15 Nu 1 1 5 Fig. 4 Verification of Nusselt number Fig. 5 Nusselt number versus ynolds number 3
1,6 4. 1 1 1,4 1, 3. 8 1, h (W/m o C) 6 8 Nu Nu / NuP 6 4. 4 Fig. 6 Heat transfer coefficient versus ynolds number 1. Fig. 7 Nu/Nu plain versus ynolds number Figure 7 depicted the proportion of Nusselt number of the tube with louvered turbulators to the plain tube (Nu/Nu plain ), plotted with ynolds number. It is clearly seen that the non-linear behaviors of all case are dominant. The Nusselt numbers are nonlinearly decreased as progressed. The proportion of Nusselt number of 31 degree of incidence and the plain tubes are greater than those of 17 and 6 degrees. The pressure drop characteristics of the tube with louvered turbulators and the plain tubes were presented in Figure 8. The more angle of attack of louvered fin is, the greater pressure drop is produced. The pressure drop value of the plain tube are lowest compared with the other. The pressure drop behavior is slightly nonlinear and increased as the ynolds number advance. The installed louvered fins act as fluid obstructers, causing the loss of flow kinetic energy, converted in the form of pressure drop. The experimental friction factors were compared with the past correlation of Petukhov [7] and Blasius [7]. In Fig. 9, both of the present work and past correlation of the plain tube results are harmonious. The friction factor is inversely proportional with ynolds number. Shown in Figs. 1 and 11, the friction factors become greater as the angle of attack is more and more increased. The maximum values of friction factor were found in the case 31 degrees of angle of attack, while the plain tube case gave the minimum. The friction factor has less influence, as ynolds number increase. The friction proportions of 31 degrees of incidence and the plain tubes are higher than those of 17 and 6 degrees of incidence for all ynolds number. Pressure drop (Pa) 3 5 15 1 5 Fig. 8 Pressure drop versus ynolds number 4
.7 f (-).6.5.4.3 Experimental Petukhov Blasius..1. Fig. 9 Verification of friction factor 1. 7..8 6. f (-).6.4 f / fp 5. 4. 3... 1.. Fig. 1 Friction factor versus ynolds number. Fig. 11 f/f plain versus ynolds number 5. CONCLUSIONS The research presented experimental study to improve the double pipe heat exchanger by installing the louvered strips. The results can be concluded as follows; Louvered strips enhanced heat transfer performance. The 17, 6, and 31 degrees of incidence of the strips augmented average heat transfer by approximately 133, 186, and 46 %, respectively for ynolds number range of 6 to 65 Louvered strips increased friction factors of the heat exchange device. The friction factors were increased approximately 119, 145, and 167 % by installing the inclined strips of 17, 6, and 31 degrees, respectively. 6. NOMENCLATURE Q Heat transfer rate (W) M Mass flow rate (kg/s) C p Specific heat (J/kg K) T h,inlet Hot water inlet temperature (K) T h,outlet Hot water outlet temperature (K) T c,inlet Cold water inlet temperature (K) T c,outlet Cold water outlet temperature (K) hf Cold water average temperature (K) T cs Local copper tube surface temperature (K) Average copper tube surface temperature (K) cs h m Convection heat transfer coefficient (W/m K) k Conduction heat transfer coefficient (W/mK) A h Heat transfer area (m ) D Diameter of copper tube (m) Nu m Mean Nusselt number h ynolds number P Pressure drop (Pa) f Friction factors ρ Hot water density (kg/m 3 ) U m Mean velocity (m/s) µ Fluid absolute viscosity (Ns/m ) 5
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