Heat Transfer Co-efficient and Effectiveness for Water Using Spiral Coil Heat Exchanger: A Comprehensive Study

Transcription

1 Global Science and Technology Journal Vol. 1. No. 1. July 2013 Issue. Pp Heat Transfer Co-efficient and Effectiveness for Water Using Spiral Coil Heat Exchanger: A Comprehensive Study M A. Hossain 1, M I. Islam 2, S A. Ratul 2 and MT U R. Erin 2 Spiral coil heat exchangers play a vital role in cooling high density and high viscous fluids. An experimental study has been conducted to investigate the overall heat transfer coefficient and effectiveness for water using spiral coil heat exchanger. A physical model of the spiral coil heat exchanger was designed, built, and instrumented for temperature measurements. The mass flow rate of hot fluid was varied from kg/sec to kg/sec and the mass flow rate of cold fluid was varied from kg/sec to kg/sec. Experiments have been conducted by varying combination of the mass flow rates of cold and hot water. The effects of relevant parameters on spiral coil heat exchanger are investigated. The result that has been achieved in the research was impressive and encouraging. Field of Research: Mechanical Engineering Keywords: Spiral Coil Heat exchanger; Reynolds number; Heat transfer coefficient; Effectiveness. 1. Introduction The heat transfer characteristics in spiral coil heat exchangers have received comparatively little attention in literature. It is a very much recent concept. However, most of the studies for compact heat exchangers are carried out with the helical coil heat exchangers and spiral plate heat exchangers. But due to lack of experimental data, the information on spiral coil heat exchangers, in open literature, is limited. There are some earliest works, regarding spiral coil heat exchangers, were performed by - Seban and McLaughlin (1963)calculated heat transfer in coiled tubes for both laminar and turbulent Flows. Plot of Nusselt vs. Graetz numbers were presented for coils with curvature ratios of 17 and 104 with Reynolds numbers ranging from 12 to 5600 for the laminar flow region. Heat transfer and pressure loss in steam heated helically coiled tubes were studied by Rogers(1964). They observed that even for a steam heated apparatus, uniform wall temperature was not obtained, mainly due to the distribution of the steam condensate over the coil surface. Mori (1965)studied the fully developed flow in a curved pipe with a uniform heat flux for large Dean Numbers. Outside-film and inside-film heat transfer coefficients in an 1 Dr. M A. Hossain, Assistant Professor, Department of Mechanical Engineering, Military Institute of Science and Technology (MIST), Dhaka, Bangladesh. ahossain@me.mist.ac.bd 2 Researchers, Department of Mechanical Engineering, MIST, Dhaka, Bangladesh. 12

2 agitated vessel were studied by Jha (1967). Prandtl numbers of , and curvature ratios of 10 to 100 for fully developed velocity and temperature fields were performed by Kalb(1974 and 1972). The effects of buoyancy forces on fully developed laminar flow with constant heat flux were studied analytically by Yao(1978). Rayleigh number and Dean number were presented for both orientations. Laminar flow and heat transfer were studied numerically by Zapryanov(1980) using a method of fractional steps for a wide range of Dean (10 to 7000) and Prandtl (0.005 to 2000) numbers. Their work focused on the case of constant wall temperature and showed that the Nusselt number increased with increasing Prandtl numbers, even for cases at the same Dean number. Heat transfer and flow characteristics in the curved tubes have been studied by a number of researchers. Goering and Humphrey (1997) solved the fully elliptic Navier Stokes and energy equations to analyze the effects of buoyancy and curvature on the fully developed laminar flow through a heated horizontal curved. Yang et al. (2000) studied the effects of the flow rate, the Prandtl number, the pipeperiod and the pipe-amplitude on the heat transfer for a laminar flow in a pipe with periodically varying finite curvature. Nigam et al. (2001) solved the governing equations of fully developed laminar flow and heat transfer of Newtonian and power law fluids in the thermal entrance region of curved tubes for analyzing the secondary velocity profile, temperature profile and Prandtl number. The mathematical model for turbulent flow based on the equations of conservation of mass, momentum and energy was solved by Targett et al. (1995). They studied the fully developed angular flow and fully developed convection in the annulus between two concentric cylinders. Li et al. (1998) applied the renormalization group (RNG) k e model for considering the three-dimensional turbulent mixed convective heat transfer. Yang and Chiang (2002) studied the effects of the Dean number, Prandtl number, Reynolds number and the curvature ratio on the heat transfer for periodically varying curvature curvedpipe inside a larger diameter straight pipe to form a double-pipe heat exchanger. Suga (2003) presented a two-component limit turbulence model and the wall reflection free model. The results showed that the two-component limit turbulence model was reliable in the case of strong curvature. Wang and Cheng (1996) numerically studied the combined free and forced convective heat transfer in a rotating curved circular tube with uniform wall heat flux and peripherally uniform wall temperature. he most productive studies were continuously carried out by Ho et al. (1995) and Ho and Wijeysundera (1996, 1999). They applied the correlations of the tube-side and air-side heat transfer coefficients reported in literature to determine the thermal performance of the spiral coil heat exchanger under cooling and dehumidifying conditions. Experiments were performed to verify the simulation results. Later, Naphon and Wongwises (2003) modified the mathematical model of Ho and Wijeysundera (1999) by including the fin efficiency and using the other existing heat transfer coefficient correlations. Recently, in order to solve the lack of the appropriate heat transfer coefficient correlations for the spirally coiled configuration, Naphon and Wongwises (2002) proposed a correlation for the average in-tube heat transfer coefficient for a spiralcoil heat exchanger under wet-surface conditions. The experiments with the same experimental apparatus were also conducted under dry-surface conditions and the average inside and outside heat transfer coefficients were proposed by Naphon and Wongwises (2003). 13

3 In the present paper, the main concern is to propose the inside heat transfer coefficients for the tube of the spirally coiled tube heat exchanger under wet-surface conditions. Correlations in the practical form are developed from all experimental data for predicting the heat transfer coefficients. 2. Methodology Figure 1 shows the experimental setup line of art schematic diagram that has been used in this research work. Fig 1: Block Diagram of Experimental Apparatus The outlet cold water flow rate is kept constant and the inlet hot water flow rate is varied using a gate valve. For the same cold water flow rate, the hot water flow rate is changed for five times. The hot water is circulated in a closed loop and the cold water in an open loop. The experiment starts with raising the temperature of hot reservoir by an electric heater capacity of around 2 kw. When the temperature of hot reservoir is raised around c, a 0.5 HP centrifugal pump is opened to circulate the hot water through the heat exchanger.at the same time cold water is supplied through the heat exchanger. Thermocouple T 1 and T 2 are used to measure inlet and outlet temperature of hot water respectively; T 3 and T 4 are used to measure the inlet and outlet temperatures of cold water respectively. For different hot water flow rates the temperatures at the inlet and outlet of hot and cold water are recorded, after achieving the steady state. This procedure is repeated for cold water flow rates and temperatures, and mass flow rates. The mass flow rate is measured by using the flowmeter. The mass flow rate of cold water is measured by bucket method. Table 1 shows the material and dimensions that have been used for design the setup. Table 2 shows the boundary condition for the experiment. 14

4 Table 1: Material and Dimension of the Setup Particulars Mild Steel Shell Copper tube Coil Water pump Flow meter Hot Reservoir Thermocouple Electric Heater Specification 350mm dia, 500mm length 9.5mm inner dia, 50 m length 0.5 hp 18 Lpm 40 liter K-type 2 kw Table 2: Boundary Conditions for the Experiment Variable Hot Water Temperature Cold Water Temperature Mass flow rate of hot water Mass flow rate of cold water Range c c kg/sec kg/sec 3. Calculation Methodology The heat released or absorbed is calculated using the expression, Q= C p ΔT (1) where, is hot or cold water flow rate, C p is specific heat capacity of water, ΔT is Temperature difference of hot and cold water. The Nusselt Number of hot water for fully developed turbulent flow inside smooth tube can be determined by using Colburn equation (0.7<Pr<160, Re> 10, 0000): Nu h = 0.023Re h 0.8 Pr 0.3 (2) where, Re h = =Reynolds Number of hot water and Pr =Prandtl Number Empirical relations have been developed for predicting Nusselt Number for laminar flow in the entrance region of a circular tube. One such correlation is given by Hausen as. Nu c = ( ) / (3) where, Gz = Gratez Number = (4) / L and D are the length and inner diameter of the shell respectively. Heat transfer co-efficient (h) for flow through a circular shell, 15

5 h= Nu. (5) where, k =thermal conductivity of water (W/m 0 C) If, h i and h 0 are heat transfer co-efficient of hot and cold water respectively then overall heat transfer coefficient (U) can be calculated using the following equation, U= (6) Logarithmic mean temperature difference (LMTD) can be found from the following equation LMTD = ( ) ( ) (( )/( )) (7) where, T 1 =Hot water inlet temperature; T 2 = Hot water outlet temperature; T 3 =Cold water inlet temperature; T 4 =Cold water outlet temperature Heat transfer area (A) is estimated from the following equation, A = Effectiveness ( ) of the heat exchanger, = ( ) (( ) (8) (9) Number of (heat) transfer unit (NTU) which is a dimensionless parameter is defined as, NTU = (10) 4. Result and Discussion The performance characteristics of spiral coil heat exchanger i.e. heat transfer coefficient (hot water), overall heat transfer coefficient, Nusselt Number, Effectiveness with respect to Reynolds Number and effectiveness, heat transfer coefficient and heat capacity with respect to hot water flow rate for three different cold water flow rates are illustrated from figure 1to 8. The effects of heat transfer rate on Reynolds number (Re) for three different cold water flow rate is shown in Figure 2. It is observed that the heat transfer rate increases almost linearly with increasing Reynolds number (Re), which is acceptable for the spiral coil heat exchanger. The heat transfer rate is maximum in case of medium cold water flow rate (0.129kg/sec). 16

6 Fig 2: Variation of Heat Transfer Coefficient with Reynolds Number for Different Cold Figure 3 shows the variation of the overall heat transfer co-efficient, U (W/m 2 K) with Reynolds Number (Re) for three different cold water flow. From this plot it is observed that, the overall heat transfer co-efficient, U (W/m 2 K) is maximum for the highest cold water flow rate (0.225kg/sec) and minimum for the lowest cold water flow rate (0.029kg/sec). Fig 3: Variation of Overall Heat Transfer Coefficient with Reynolds Number for Different Cold Overall Heat Transfer Coeffient, U (W/m 2 k) E+00 1.E kg/s 0.129kg/s 0.029kg/s 2.E+04 3.E+04 4.E+04 5.E+04 6.E+04 Reynolds Number (Re) 7.E+04 8.E+04 Figure 4 shows the plot of the Nusselt Number (Nu) with Reynolds Number (Re) for three different cold water flow rate. From the experimental result it is shown that the Nusselt number increases linearly with increasing of Reynolds Number for all three cold water flow rate. At 0.129kg/sec cold water flow rate Nu is maximum with respect to Re. 17

7 Fig 4: Variation of Nusselt Number with Reynolds Number for Different Cold Nusselt Number (Nu) E+00 2.E+04 4.E+04 6.E+04 Reynolds Number (Re) 0.225kg/s 0.129kg/s 0.029kg/s 8.E+04 Figure 5 shows the plot of heat transfer effectiveness ( ) with respect to Reynolds Number for three different cold water flow. From this plot it is observed that, the effectiveness ( ) decreases with the increase of Reynolds Number (Re) and the value is maximum for the medium cold water flow. It is occurred because at higher Reynolds Number the hot and cold water get less time to exchange the heat between them. As a result effectiveness is lower for the heat exchanger. Fig 5: Variation of Effectiveness with Reynolds Number for Different Cold Effectiveness (ɛ) k g/s 0.129k g/s 0.E+00 1.E+04 2.E+04 3.E+04 4.E+04 5.E+04 6.E+04 7.E+04 8.E+04 Reynolds Number (Re) Figure 6 shows the variation of heat transfer effectiveness ( ) with respect to the hot water flow rate for three different cold water flow rate. From this graph we observed that, the effectiveness ( ) decrease with the increase of hot water flow and it is maximum for the medium cold water flow. It is occurred because at higher hot water flow rate the hot and cold water get less time to exchange the heat between them. As a result effectiveness is lower for the heat exchanger. Figure 7 shows the plot of overall heat transfer co-efficient, U (W/m 2 K) with respect to the hot water flow rate for three different cold water flow rate. From this plot it is observed that, the overall heat transfer co-efficient, U (W/m 2 K) is maximum for the highest cold water flow rate (0.225kg/sec) and it is minimum for the lowest cold water flow rate (0.029kg/sec). 18

8 Fig 6: Variation of Effectiveness with Hot Water Flow Rate for Different Cold Fig 7: Variation of Effectiveness with Hot Water Flow Rate for Different Cold Figure 8 shows the scatter plot of Heat Absorbed by the cold water with respect to Hot water flow rate for three different cold water flow rate. From this graph it is observed that, heat absorbed by the cold water is decreased with the increased of hot water flow rate and heat absorbed is maximum for highest cold water flow rate and minimum for lowest cold water flow rate. Figure 9 shows the plot of heat absorbed by the cold water with respect to Reynolds Number (Re) for three different cold water flow rate. From this plot it is observed that, heat absorbed by the cold water decreases with the increase of Reynolds Number (Re) and heat absorbed is maximum for higher cold water flow and minimum for lower cold water flow. It is because when Re is high the cold water gets less time to absorbed heat from hot water. 19

9 Fig 8: Variation of Heat Absorbed with Hot Water Flow Rate for Different Cold Fig 9: Variation of Heat Absorbed with Reynolds Number for Different Cold 5. Conclusions This paper presents the comprehensive study on heat transfer characteristics and the performance of a spiral coiled heat exchanger. The results have explained the better understanding of heat transfer co-efficient and effectiveness of a spiral coil heat exchanger, which is newly attraction for the researcher. The heat transfer rate depends directly on mass flow rate of hot and cold water in which maximum heat transfer rate is obtained at lower hot water flow. The heat transfer coefficient is increased with the increase of both Reynolds Number and hot water flow whereas the effectiveness is inversely proportional to the hot water flow and the optimum effectiveness is obtained when the temperature difference between hot and cold water is maximum. For limited opportunity to survey of previous work on spiral coil heat exchanger, the some degree of judgment was also restricted for the researchers. As this is a newly field of research this work will contribute mostly for further study. The effectiveness, heat transfer coefficient, Nusselt number, and heat absorption can also be calculated for air-air and/or water-air flow for same setup. 20

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