International Journal of Mechanical and Production Engineering Research and Development (IJMPERD) ISSN 2249-6890 Vol. 3, Issue 2, Jun 2013, 137-146 TJPRC Pvt. Ltd. STUDY, ANALYSIS AND DESIGN OF AUTOMOBILE RADIATOR (HEAT EXCHANGER) PROPOSED WITH CAD DRAWINGS AND GEOMETRICAL MODEL OF THE FAN CHAVAN D. K 1 & TASGAONKAR G. S 2 1 Ph.D Scholar, JJTU Rajasthan, Professor Mechanical Engineering MMCOE, Pune, Maharashtra, India 2 Professor Navasahyadri Group of Institutions Pune, Maharashtra, India ABSTRACT The heat exchanger, used in refrigeration unit, air conditioning unit, radiator used with IC engine automobiles is either rectangular or square in shape. But the air blown/sucked by the fan is in circular area developing low velocity zones or high temperature regions are created in the corners. Different heat exchangers/radiators are studied; Radiator is designed, Calculations are done, CAD drawings of radiator and geometrical model are developed. Also power consumed by fan is studied. Experimentally it is found that the power consumed by fan is 2 to 5% of power produced by engine. It is proposed to have circular heat exchanger for refrigeration, air conditioning unit and for car radiators for maximum efficiency. Till now no significant work has been carried out on circular heat exchanger and radiators. KEYWORDS: Heat Exchanger, Radiator, Cad Model of Radiator, Geometrical Model of Fan, Power etc INTRODUCTION The proposed work relates to an improved heat exchanger (Radiator) design for either heating or cooling of a fluid. Also, it deals with the work particularly which relates to an improved fan assisted air-cooled heat exchanger used in Automobiles, Internal Combustion (IC) engines, Refrigeration system, and Power plants. Different types of heat exchangers are known, in which air is used as heat transfer medium as it is freely and abundantly available, without any disposal issues. In known heat exchangers, flow of air is induced naturally or is aided by the use of one or more fans. The use of fan reduces the size and the cost of the equipment, which makes it more compact. Hence, fan assisted air cooled heat exchangers are more popular than others. In known air-cooled heat exchangers, the fan either forces or draws the air through the heat exchanger, some of which are described herein below by way of examples. In present system, the fans are placed behind the heat exchangers to force/ draw the atmospheric air. These exchangers use a shroud. This directs the air over the entire area of the heat exchanger. A study was undertaken to find out the distribution of airflow and variation of its temperature [1]. Generally, all conventional heat exchangers are either square or rectangular in shape and the fans with circular blades are used to create the flow of air through them. For the present heat exchangers, there are several drawbacks or disadvantages which can be classified as follows Fans with circular blades deliver air in a circular area even when the heat exchangers are square in shape. The velocity of the air flow generated by the fan is not constant or uniform along its entire axial direction. It is almost zero at the centre and gradually increases at the rate of square of the radius.
138 Chavan D. K. & Tasgaonkar G. S When the thickness of the heat exchanger is constant, there has been no attempt to increase the heat transfer area at the periphery of such heat exchanger. The heat transfer area near the hub of the fan should be zero. Since this is not the case with present heat exchangers, they do not offer optimum utilization of material and air velocity. A square or rectangular shrouds were provided for the fan to convert the circular flow of air into the required shape. Further the known equipments consume more power, more material and are therefore not cost effective [3]. Therefore it has been proposed to develop a new heat exchanger, which would avoid all the disadvantages of the known equipments [2]. POWER CONSUMED BY FAN The automobile radiator sometimes needs additional airflow through it to prevent the engine from overheating. This usually occurs at idle and slow speed. At higher vehicle speeds, the air flows through the radiator by the forward motion of vehicle provide all the cooling that is needed. An engine fan or cooling fan pulls the additional air through the radiator. The fan may be either a mechanical fan or an electric fan. Engines mounted longitudinally in rear- drive vehicles usually have a mechanical fan that mounts to the water pump shaft. The fan is made of sheet steel or moulded plastic. It has four to seven blades and turns with the water pump impeller. A fan shroud around the fan directs the airflow. This increases the efficiency of the fan. Transverse engines in front-drive vehicles usually have an electric fan. An electric motor turns the blades. A thermostatic switch turns on the fan only when needed. Generally, the switch turns on the fan when the coolant reaches 70 0 80 0 C. It turns off the fan if the coolant drops below this temperature. On the experimental trial setup of Petrol Engine and Diesel Engine, trials are conducted in the college laboratory with fan and without fan. It is observed that power consumed by the fan is of considerable magnitude and is about 2% to 5% of total power developed by the engine [4]. Any saving in the fan power is directly the saving of precious fuel. Also BHP of the engine will be mentioned by the manufacturer or it can be calculated. Also it is observed from following examples that, Cummins engine make, 1645 BHP required 42 HP for fan i.e. 2.55% of engine power. Cummins engine make, 600 BHP required 17 HP for fan i.e. 2.83% of engine power. COST OF RADIATORS Smaller radiators may cost less but will consume more power; hence we have to optimize the design. Cost of radiators, heat transferred per unit area by radiator varies according to size, capacity and materials used for radiator and fins. Standard materials generally used are aluminum, copper, steel, alloys etc. as per the cost and capacity. EXISTING RECTANGULAR / SQUARE AND OTHER RADIATOR Rectangular Radiator
Study, Analysis and Design of Automobile Radiator (Heat Exchanger) 139 Proposed with Cad Drawings and Geometrical Model of the Fan Figure 2: Rectangular Radiator This existing design is most popularly used in the current applications. As shown in figure 2 hot water is allowed o flow through the inlet port, to upper tank where the hot water is distributed through a system of tubing. These tubes are surrounded and connected by a number of fins along the entire length of said tubes as shown. A fan is mounted on a shaft which causes a circulation or draught of air through the radiator and the fins. A shroud is adopted to regulate the flow of air from the fan so that it minimizes the quantity of air flow to escape. The atmospheric air collects the heat from the hot water as it flows over said system of tubes. The fins provided over the tubes increases the heat transfer area. The water entering through the inlet is allowed to flow the system of tubing before it gets cooled by the air and ultimately it comes out through an outlet port, after passing through the lower tank as shown. Heat exchanger for air conditioner Figure 3: Heat Exchanger for Air Conditioner Figure 3 shows a heat exchanger used in air conditioner, where the refrigerant enters the heat exchanger through an inlet port and it passes through a system of tubing. The flow of refrigerant takes place through the tubing. The tubing transfer heat to the air through a series of fins attached to the tubings. The refrigerant ultimately escapes out through an outlet port. As shown figure 3, in this case also a fan is provided to facilitate the air flow for enhancing the cooling / heating of refrigerant. Square Shaped Heat Exchanger This also is one of the constructional types of radiator existing in current market [5] Figure 4: Air-Cooled Square Shaped Heat Exchanger
140 Chavan D. K. & Tasgaonkar G. S Figure 4 shows a square-shaped heat exchanger with a fan provided to deliver air in a circular area. If the length and breadth of the heat exchanger is equal to D, the effective area of such heat exchanger will be equal to D 2. While the flow of air from the fan (without shroud) will be of area (π/4) D 2 = O.76 D 2. The difference in the area of the square and the circle would be {D 2 - (π/4) D 2 } = 0.24 D 2. OBJECTIVES OF PRESENT WORK To optimize the fan assisted heat exchanger (radiator) by improvement in design. To provide a heat exchanger that will be more efficient and compact. To provide a heat exchanger that will work with minimum power consumption for the fan and with maximum utilization of air flow. To have a heat exchanger with minimum material and will thus be less costly. Excluding the central hub area, the material saving is @ 24%, saving in the cost of production on mass scale basis once the dies are manufactured will be about 20 %. Considering the number of vehicles, refrigerators and air conditioners used at national and international levels, slight improvement in efficiency and reduction in cost will add to the economy to a great extent. CAD MODEL OF RADIATOR Material of tubes: Aluminium Number of tubes: 20 Inner Diameter of Tube: 10mm Outer diameter of tube: 11.25 mm Material of fin: Aluminium Thickness of fin: 0.16 mm GEOMERIC MODELING OF FAN Figure 5: Radiator CAD Drawing For CFD analysis of the radiator, it is necessary to create a geometric model of the system. A model of the radiator and the fan was made in CATIA V5 and then exported to CFD analysis software.
Study, Analysis and Design of Automobile Radiator (Heat Exchanger) 141 Proposed with Cad Drawings and Geometrical Model of the Fan PROPOSED HEAT EXCHANGER Figure 7: Fan Model Figure 8: Radiator Model To overcome the drawbacks of this current and conventional design, a new design with geometrical modifications is proposed to obtain efficient working of the radiator. Figure 9: Proposed Circular Heat Exchanger (Radiator) In the existing design, there is no heat transfer area at the centre where the flow of air is almost zero. In the proposed design the tubes and the fins are so arranged that the outlet air has nearly constant velocity [6]. Heat exchanger for the refrigeration air conditioning system Figure 11: Heat Exchanger for the Refrigeration Air Conditioning The Heat exchanger for the refrigeration air conditioning system as shown in figure 11 is provided with a number of fins of varying depth, maximum depth at the outer periphery and the reducing depth along the inner periphery. It is
142 Chavan D. K. & Tasgaonkar G. S found that the new heat exchanger as described herein above has better performance level in its application. Further it consumes less power and it is thus more economical. DESIGN CALCULATIONS FOR RADIATOR Available Data Table 3: Observations of Air and Water Sr. No. Observations Air (Cold) Water (Hot) 1 Inlet Temperature ( C) 28 52 2 Outlet Temperature ( C) 34.376 44 3 m i.e mass flow rate (kg/hr) 525.35 100 4 Cp. Specific Heat (kj/kg C) 1 4.187 5 K Thermal Conductivity (W/mK) 0.024 0.66 6 Density (kg/m 3 ) 1.1 1000 Assume: For this air cooled heat exchanger we use aluminum tubes of following dimension, Outer Diameter = 11.25 mm Inner Diameter = 10.00 mm 3. Thickness = 1.25 2 = 0.0625 mm From the chart of typical values of overall heat transfer coefficient, we know that for air cooled heat exchanger value of overall heat transfer coefficient (U) ranges from 300-450 W/m 2 K. So here we assume it to be equal to 350 W/m 2 K[42]. U=350 W/m 2 K Using Energy balana equation, ( mc p ) h (T hi T u ) = ( m C p ) c (T ce T ci ) 100 4.187 (52 44) = 525.35 1 (T ce 28) T ce = 34.376 C So outlet temperature of air is = 34.746 C. We know that, q = m w Cp w T w q = 100 4.187 (52 44) q = 3349.6 Watt Assuming the Heat Exchanger (Radiator) to be counter flow we get,
Study, Analysis and Design of Automobile Radiator (Heat Exchanger) 143 Proposed with Cad Drawings and Geometrical Model of the Fan 1 =17.624 C 2 = (44 28) 2 =16 C Substituting these values in equation (5.10) we get, m = 1 2 ln 1 2 m = (17.624 16) ln (17.624 / 16) m = 16.8 C i.e. 289.8 K i.e. LMTD=16.8 C Now, using the average velocity of water in tubes and its flow rate the total flow area is given as, A f = m V Where, A f = Total flow area V = Average velocity of water = Density of water Here, we have average velocity of water = 65 m/hr V=65 m/hr So we get, A f = 100 65 1000 A f = 1.538 10 3 m 2 But we know that, A f = n 4 d2 i Where, n = Number of tubes d i = Inlet diameter of tube
144 Chavan D. K. & Tasgaonkar G. S Substituting respective values, we get 1.538 10 3 = n 4 (10 10 3 ) 2 After solving the above equation we get, n = 19.582 approximate = 20 n=20 For correction factor required dimension parameters are, P = (T ce T ci ) (T hi T ci ) P = 34.376 28 52 28 = 0.3985 R = (T hi T he ) (T ce T ci ) R = 52 44 34.376 28 = 1.26 So referring the chart 5.14, we get value of correction factor as 0.96, F=0.96 That area of the heat transfer after considering correction factor is given as, A = A = q U F m(counterflow) 3349.6 350 0.96 16.8 A=0.5934 m 2 Final Acceptable Design Parameters are as Under Number of tubes per pass = 20 Number of passes = 1 Length of tube per pass = 0.284 m Effectiveness of Heat Exchanger (I) C h = (m Cp) water C h = (100 4.18 1000) 3600 C h =116.306 W/K
Study, Analysis and Design of Automobile Radiator (Heat Exchanger) 145 Proposed with Cad Drawings and Geometrical Model of the Fan (II) C c = (m C p ) air C c = (525.35 1 1000) 3600 C c = 145.931 W/K i.e C min = 116.306 W/K C max = 145.931 W/K Now (a) Capacity ratio (C): C = C min C max C = 116.306 145.931 C = 0.797 (b) NTU : NTU = U A C min NTU = 350 0.5934 116.306 NTU = 1.786 Using NTU- correlation for cross flow HE with both fluids unmixed, we have =1 exp =1 exp 1 C (NTU)0.22 {exp [ C (NTU) 0.78 ] 1} 1 0.797 (1.786)0.22 {exp [ 0.797 (1.786) 0.78 ] 1} =0.6388 So we get effectiveness equal to 0.6388 LIMITATIONS Bending of tubes in circular shape. Inserting of fins in the tubes. Dies are to be manufactured for circular radiators which are exorbitantly costly. CONCLUSIONS Low velocity zones and high temperature regions (low heat transfer regions) are identified in corners we observe that velocity increases with the increase in rpm of radiator fan. For optimum efficiency eliminate corners and develop radiator of Circular shape.
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