Influence Of Coiled Wire Presence On The Convective Heat Transfer Coefficients In R-600 CondensersSeyed Ali Agha Mirjalily1, Aboulghasem Chakerizade2,*, Amir Javad Ahrar31 Department of Mechanical Engineering, Yazd Branch, Islamic Azad University, Yazd, Iran, Email: [email protected],* Master of Mechanical Engineering, Gas Company of Yazd Province, Yazd, Iran, Email:[email protected] Faculty of Engineering, Mechanical Engineering Department, Ferdowsi University of Mashhad, Email:[email protected] the present study, an experimental investigation has been performed to examine the heat transfer enhancement achieved by helically coiled wire inserts during forced convective condensation of R-600 vapor inside horizontal tubes. The experimental set-up is a vapor compression refrigeration cycle equipped with all the required instrumentation which was designed and constructed in the laboratory. This system includes a main condenser which is a coaxial double-pipe counter-flow heat exchanger which utilizes water as a coolant. To reach the favorable vapor qualities, pre-condenser and after-condenser were invoked. Experiments were carried out for plain tube and five different coiled wire inserted tubes. At each test run, various parameters were measured like refrigerant mass flow rate, inlet and outlet water temperature of the condenser, refrigerant pressure, etc. Using the acquired data, heat transfer coefficients were calculated for different tubes. Investigation of the results showed that increasing the diameter and reducing the coil pitch enhance heat transfer. Also, it was seen that employing coiled wire inserts inside tube increases the average heat transfer coefficient up to approximately 80% above the plain tube values.Keywords: Coiled wire; Enhanced heat transfer; R-600 refrigerant; Condenser.1. IntroductionMany novel and fruitful methods have been used during the last decades to enhance the heat transfer rate of heat exchangers. These progressing techniques can lead to better heat transfer efficiency and consequently, lower costs and smaller sizes of the heat exchangers are achieved. Due to the expensive costs of energy and lower living area limits, the research on the topic is accelerated in the recent years. Especially, in lots of industries which suffer the lack of space area and are in need of strong cooling systems, keeping the cooling procedure costs in an affordable and reasonable range is a must 1-6. Condensers are considered as one of the most applicable types of heat exchanging devices. In an industry usually one can find condensers in steam power plants, HVAC systems, and chemical industries. As an example, in HVAC systems due to the wetting characteristics of the coolants, only the layer condensation is observed, and since the thermal conductivity of the coolants is usually low, the total entropy generation of the system is reduced. Therefore, these enhancing methods of heat transfer are even more essential in HVAC systems.Akers et al. 7 investigated the influence of the vapor velocity, temperature difference, liquid load and fluid properties on the mean heat transfer coefficient of condensing vapor in a horizontal tube. They replaced the vapor with a liquid of the same surface tension on the vapor-liquid contact line in a theoretical analysis. Boyko and Kruzhilin 8 proposed a correlation to predict the mean heat transfer coefficient during the condensation process of water vapor. Soliman 9 studied the influence of mist-annular transition on the heat transfer process of condensation of vapor. He indicated that using annular correlations for mist region is not fully accurate, and to calculate the deviation of flow characteristics from the annular region, one can apply for the Weber number. He also proposed an experimental correlation to estimate the heat transfer rate in mist regions. This equation was formed under the assumption of the homogeneous vapor-liquid mixture. Royal and Bergles 10 investigated the effects of tube inclination angle, internal fins and twisted tape on the condensation of vapor in tubes. They discovered that the tube inclination angle does not necessarily affect the heat transfer rate, but on the other hand, the twisted tapes can enhance the heat transfer coefficient of the horizontal condenser by 30% with respect to a plain tube. Moreover, the straight and spiral fins increased the heat transfer rate up to 150%. Akhavan Bahabadi et al. 11 studied the influence of applying twisted tape in R-134a condensers. They reported significant heat transfer enhancement in the system.Salimpour and Yarmohammadi 12, 13 studied the same phenomenon using R-404a refrigerant. In another experimental research, Salimpour and Golami 14 investigated the pressure drop due to the condensation of R-404a in the presence of coiled wire and reported a significant increment in heat transfer rate. Agrawal et al. 15 also, used the coiled wire in a tube filled with R-22 refrigerant and studied its condensation process. Mostly, their achievements indicated spiral and semi-spiral flow patterns in the tube. In addition, they observed oscillation in the temperature of the tube wall. Moreover, they reported an increment up to 100% in the heat transfer regarding the plain tube. In this study, it is shown that the heat transfer coefficient is a complicated function of the refrigerant vapor Re number and the coiled wire pattern. They also observed better heat transfer enhancement in the presence of coiled wire with respect to the tape twisted tubes. Akhavan Behabadi et al. 16 investigated R-134a condensation in a plain tube and a tube equipped with coiled wire, experimentally. They declared that the heat transfer coefficient is a function of vapor Re number and the coiled wire pattern again. In their investigations, an 80% increase in the heat transfer rate is reported as well. To the best of our knowledge, the influence of the coiled wire presence on condensation process of R-600 vapor has not been considered as a case study by researchers yet. Consequently in this study, for the first time, the condensation process of R-600 is investigated experimentally, and the influence of the coiled wire patterns on the forced convection and condensation of R-600 is studied. 2. Experimental setup The designed experimental setup is a vapor-condensing refrigerator system consisting all necessary sensors and test devices. A schematic view of the test setup is illustrated in figure 1. It is obvious from this figure that the main condenser is consisting of three subdivisions: the primary condenser, the test condenser, and the secondary condenser.The test condenser is a double pipe counter-flow heat exchanger. R-600 refrigerant is condensed in the inner tube and exerts heat to the water flowing in the outer tube. The inner tube is made of copper with 14.1 mm internal diameter, 0.9 mm thickness and 1 m length, while the outer tube is made of steel with 80 mm internal diameter. The flow temperature is measured in 4 individual middle sections of the tube on both top and bottom sides. The sensors for temperature are chosen to be the J-type thermocouples and four additional sensors are placed at the entrance and exit locations of the tube. Also, the temperature of the coolant water is recorded at the inlet and outlet sections. Figure 1: A schematic diagram of the experimental setupMoreover, the refrigerant flow absolute pressure is observed at the inlet and outlet sections of the test condenser via pressure gauges. Also, the whole test condenser is isolated by glass wool to prevent any undesired heat loss from the system. In order to have a suitable and adjustable vapor quality at the entrance of the test condenser, a primary condenser is used. This primary condenser provides us the ability to regulate and adjust the vapor quality by altering the coolant (which in this case is water) flow rate, and consequently one can monitor the influence of altering refrigerant quality on the heat transfer rate and pressure drop of the system. This condenser is a double tube heat exchanger in which the cold water flows in the outer tube and the refrigerant flows through the inner pipe. The primary condenser size is the same as the test condenser, with only one difference that the copper tube length is 9 m and is swirled 9 times. The outer and inner diameters of this condenser are 9.525 mm and 7.525 mm, respectively. To increase the heat transfer capacity of the system a coiled wire is used in the test section. This additional coiled wire may have 3 various diameters of 0.5, 1 and 1.5 mm and 3 different pitches of 8, 10 and 13 mm.3. Experimental discussion After designing the experimental setup and installing the system in a lab, the required test instruments were inserted. Afterward, the test instruments were calibrated with the predefined format, and the system went through several validation tests. After correctness and exactness insurances the real tests and data refinements were carried out. During the test 3 datasets in 10 min intervals were recorded after the system steady state. The average value of these 3 records was applied for the rest of calculations. The recorded data are as follows:*. The mass flow rate of the refrigerant*. The mass flow rate of the coolant water in the test condenser, primary condenser, and secondary condenser*. The refrigerant temperature at the inlet and outlet of the test, primary and secondary condensers*. The temperature of outer walls of the inner tube of the test condenser*. The temperature of the coolant water at entrance and exit sections of the test, primary and secondary condensers*. Static pressure of the refrigerant at the inlet and outlet of the test, primary and secondary condensersA total number of 180 individual tests with 5 different mass flow rates and 6 various vapor qualities for plain and coiled wire equipped tubes were carried out. At first, the experimental data for plain tube were achieved, then the system was reset with the mentioned coiled wire and the experiments were carried out again for the new setup. 4. Calculations and formulationsIn each test case, the heat transfer coefficient is obtained based on the absorbed heat by the coolant water in the test section and the temperature difference between the condensing refrigerant and the tube walls. In order to calculate the heat transfer coefficient, firstly the condenser outer wall average temperature Tws is estimated as: (1)In which TT and TB refer to the temperature of the top and bottom sides of the tube. Then the mean temperature of test condenser walls Two is obtained from the 4 estimated average temperatures: (2)The exerting heat by the coolant water is calculated from: (3)Here, is the coolant water mass flow rate and Cpw is the thermal capacity of water. The radial heat flux can be computed then as: (4)The temperature drop in the tube wall is also related to the radial heat flux and conduction heat transfer coefficient of the copper as: (5)Thus the inner wall mean temperature is obtained from: (6)The average static pressure in the test section is estimated as the average of inlet and outlet pressure gauges, and since the vapor is in the saturated region the mean temperature of the vapor is assumed to equal the temperature related to this static pressure. Finally, the heat transfer coefficient of the test condenser is calculated from: (7)In which, Ts is the saturated temperature of the vapor. 5. Results and discussionsThe condensing heat transfer coefficient for R-600 flow in both coiled wires equipped tube and plain tube in different refrigerant situations is investigated experimentally. The vapor quality is estimated from the average of vapor qualities at inlet and outlet sections of the test condenser. Then the influences of certain parameters like refrigerant quality and mass velocity, coolant mass flow velocity and the coiled wire specifications are studied in the condensation process and heat transfer rate of the system. Figure 2 aims to elicit the influence of the vapor quality on heat transfer coefficient. In this figure, the heat transfer rates for 5 different mass velocities are presented. Moreover, for each of these mass velocities, 6 different tests with various vapor qualities were carried out under the same condensing pressure, and hence a wide range of vapor quality is covered. This figure mostly indicates that if all other parameters remain untouched, the heat transfer coefficient augments with the mass velocity increment. In fact, higher refrigerant velocities lead to a more turbulent flow regime in the condensing layer, and consequently, it would accelerate the condensation process. On the other hand, it is obvious from figure 2 that reduction of vapor quality in the test condenser results in a certain decrement of heat transfer rate. This phenomenon also is rooted from the fact that in higher vapor qualities, the liquid refrigerant layer on the inner walls of the tube is thinner and they will cause less thermal resistance on the wall. Figure 2: Variation of the heat transfer coefficient versus the vapor quality for different mass velocities.Experimental results for the coiled wire equipped tube for 3 different wire diameters of 0.5, 1 and 1.5 mm with 10 mm pitch were recorded. Also, 3 various pitches of 8, 10 and 13 mm were tested with 1.5 mm wire diameter. The specifications of these coiled wire equipped tubes are presented in table 2. In this table, D is the inner diameter of the tube, Do is the outer diameter, De is the effective diameter, L is the length of the tube, e is the coiled wire thickness P is the coil pitch and is the swirling angle.Table 1: dimensions and specifications of the coiled wire equipped tubesdegree ? P mm e mm L mm De mm Do mm D mm Tube set73 10 0.5 1000 11.9 15.9 14.1 A73 10 1 1000 10.4 15.9 14.1 B73 10 1.5 1000 9.1 15.9 14.1 C76.6 8 1.5 1000 8.4 15.9 14.1 D69 13 1.5 1000 9.9 15.9 14.1 E— — — 1000 — 15.9 14.1 F Figure 3: Variation of the heat transfer coefficient versus the vapor quality for coiled wire equipped and plain tubes of A, B, C, and F at G=142.4 Kg/m2s.Figure 3 illustrates the influence of coiled wire thickness on the heat transfer coefficient. As can be seen, addition coiled wire in the tube results in an outstanding heat transfer enhancement. The highest increment was observed in the tube equipped with the thickest coil, i. e. tube D. this coiled wire can increase the mean heat transfer coefficient of the system at G=142.4 ( ) up to 80% regarding a plain tube. Figure 4: Variation of the heat transfer coefficient versus the vapor quality for coiled wire equipped and plain tubes of C, D, E, and F at G=71.2 Kg/m2s.Moreover, the behavior of heat transfer coefficient with variations of coiled wire pitch is presented in figure 4. This figure indicates that as the coiled wire thickness augments the heat transfer rate of the condenser increases as well. On the other hand, when the coil pitch is decreased, the heat transfer rate would increase. This phenomenon occurs due to the increment of flow friction with reduction of the coiled wire pitch. This friction would increase the flow turbulence and consequently, it increases the heat transfer rate. Finally, figure 5 demonstrates the variations of heat transfer coefficient versus vapor quality in tube B for five different mass velocities. This figure indicates that the heat transfer coefficient reduces by the vapor quality reduction in all mass velocities and coiled wire patterns. This happens due to the reduction of liquid layer thickness on the tube walls in higher refrigerant qualities which may cause less thermal resistance on the tube wall. In addition, one can deduce from this figure that when all other parameters are fixed, generally the heat transfer coefficient would increase with an increment of the mass velocity. This behavior is well-predicted and is in absolute accordance with the flow an heat transfer rates of the plain tube. Figure 5: Variation of the heat transfer coefficient versus the vapor quality for different mass velocities in pipe B.6. Conclusions The flow and heat transfer characteristics of R-600 refrigerant condensation was experimentally investigated in a plain and several coiled wire equipped tubes. The following results were obtained from the experiments:1. The heat transfer coefficient is increased on the tube walls with an increment in mass velocity of the refrigerant, vapor quality and mass velocity of the coolant liquid.2. It was observed that the coiled wire can enhance the heat transfer rate up to 80% with regard to the plain tube. 3. It goes without saying that the heat transfer coefficient is strongly related to the geometry and sizes of the coiled wire.4. With an increment of the coiled wire diameter, the heat transfer coefficient would increase when all other parameters are fixed, and therefore the highest rate of heat transfer was observed in the tube containing the coiled wire with the biggest diameter. 5. Also, when the coiled wire pitch is reduced, the heat transfer rate augments. 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