CFD ANALYSIS OF DOUBLE HELICAL PIPE PARALLEL& COUNTER FLOW HEAT EXCHANGER

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1 CFD ANALYSIS OF DOUBLE HELICAL PIPE PARALLEL& COUNTER FLOW Abstract HEAT EXCHANGER 1 Hepsiba Sudarsanam, 2 Dvsrbm Subhramanyam 1 PG Scholar, Department of MECH, Nalanda Institute of Technology, Kantepudi,Sattenapalli Dist.: Guntur, A.P, India,Pin: Id: hepsi.sudarsanam@gmail.com 2 Asst professor, Department of MECH, Nalanda Institute of Technology, Kantepudi,Sattenapalli A heat exchanger is a device that is used to transfer thermal energy (enthalpy) between two or more fluids between a solid surface and a fluid, or between solid particulates and a fluid, at distinctive temperatures and in thermal contact. Heat exchangers are important engineering devices in many process industries since the efficiency and economy of the process largely depend on the performance of the heat exchangers. A helical coil heat exchanger has a wide range of application in industries over the straight and shell type heat exchangers because of its greater heat transfer area, mass transfer coefficient and higher heat transfer capability, etc. The relevance of helical coil heat exchanger has been identified in industrial application like turbine power plants, automobile, aerospace, etc. because of above mentioned factors. Double helical pipe is modeled by using solid works 2016 software & CFD analysis has been done for varying inlet condition keeping the heat flux of outer wall constant. Steel was used as the base metal for both inner and outer pipe and simulation has been done using ANSYS The software ANSYS 14.5 work bench was used to plot the temperature contour, velocity contour and total heat dissipation rate taking Dist.:Guntur,A.P, India,Pin: Id:subhramanyasharma@gmail.com cold fluid at constant velocity in the outer tube and hot fluid with varying velocity in the inner one. Water was taken as the working fluid for both inner and outer tube. Aim of the Present Work The design of a helical coil tube in tube heat exchanger has been facing problems because of the lack of experimental data available regarding the behavior of the fluid in helical coils and also in case of the required data for heat transfer, unlike the Shell & Tube Heat exchanger. So to the best of our effort, numerical analysis was carried out to determine the heat transfer characteristics for a double-pipe helical heat exchanger by varying the different parameters like different temperatures and diameters of pipe and coil and also to determine the fluid flow pattern in helical coiled heat exchanger. The objective of the project is to obtain a better and more quantitative insight into the heat transfer process that occurs when a fluid flows in a helically coiled tube. The study also covered the different types of fluid flow range extending from laminar flow through transition to turbulent flow. The materials for the study were decided and fluid taken was water and the material for the pipe was taken to be steel for its better conducting properties

2 Boundary conditions Cold Inlet velocity: 2 m/s Hot inlet velocity: 1.8 m/s Cold inlet temperature: 303 k Hot inlet temperature: 353 k Cold outlet: pressure outlet Hot outlet: pressure outlet Hot & cold fluid: water Inner & outer Pipe material: steel Introduction A heat exchanger is a device used to transfer heat between one or more fluids. The fluids may be separated by a solid wall to prevent mixing or they may be in direct contact. They are widely used in space heating, refrigeration, air conditioning, power stations, chemical plants, petrochemical plants, petroleum refineries, natural-gas processing, and sewage treatment. The classic example of a heat exchanger is found in an internal combustion engine in which a circulating fluid known as engine coolant flows through radiator coils andair flows past the coils, which cools the coolant and heats the incoming air. Heat exchangers are one of the mostly used equipment in the process industries. Heat exchangers are used to transfer heat between two process streams. One can realize their usage that any process which involve cooling, heating, condensation, boiling or evaporation will require a heat exchanger for these purpose. Process fluids, usually are heated or cooled before the process or undergo a phase change. Different heat exchangers are named according to their application. For example, heat exchangers being used to condense are known as condensers, similarly heat exchanger for boiling purposes are called boilers. Performance and efficiency of heat exchangers are measured through the amount of heat transfer using least area of heat transfer and pressure drop. A better presentation of its efficiency is done by calculating over all heat transfer coefficient. Pressure drop and area required for a certain amount of heat transfer, provides an insight about the capital cost and power requirements (Running cost) of a heat exchanger. Usually, there is lots of literature and theories to design a heat exchanger according to the requirements. The most important fluid flow heat exchangers are HVAC, process industry, refrigeration etc. The purpose of constructing a heat exchanger is to get an efficient method of heat transfer from one fluid to another, by direct contact or by indirect contact. There are three mode of heat transfer 1.Conduction 2.Convection 3.Radiation.Heat transfer is negligible in radiation as compare to conduction and convection. Conduction takes place when the heat from the high temperature fluid flows through the surrounding solid wall. The conductive heat transfer can be maximized by selecting a minimum thickness of wall of a highly conductive material. But convection is plays the major role in the performance of a heat exchanger. Forced convection in a heat exchanger transfers the heat from one moving stream to another stream through the wall of the pipe. The cooler fluid removes heat from the hotter fluid as it flows along or across it Heat exchangers are important engineering devices in many process industries since the efficiency and economy of the process largely depend on the performance of the heat exchangers. High performance heat exchangers are, therefore, very much required. Improvement in the performance may result in the reduction in the size of the heat exchangers of a fixed size can give an increased heat transfer rate, it might also give a decrease in

3 temperature difference between the process fluids enabling efficient utilization of thermodynamic availability. This is particularly true for Laminar flow since the heat transfer coefficients for laminar straight flow through a plain tube is very low. Forced convection heat transfer in doubly connected ducts bounded externally by a circle and internally by a rectangular polygon of various shapes was analyzed using a finite element method. Classification of heat exchangers Types of Heat Exchangers There are many types of heat exchangers. Some of them are discussed here. Shell and Tube Heat Exchanger Shell and tube heat exchangers consist of series of tubes. One set of these tubes contains the fluid that must be either heated or cooled. The second fluid runs over the tubes that are being heated or cooled so that it can either provide the heat or absorb the heat required. Fig3.1: Shell and Tube Heat Exchanger Plate Heat Exchangers Another type of heat exchanger is the plate heat exchanger. These exchangers are composed of many thin, slightly separated plates that have very large surface areas and small fluid flow passages for heat transfer. Advances in gasket and brazing technology have made the plate-type heat exchanger increasingly practical. Fig: 1.1 Classifications of Heat Exchangers Due to the large number of heat exchanger configurations, a classification system was divided based upon the basic operation, construction, heat transfer, and flow arrangements. Recuperates and regenerators Transfer processes: direct contact or indirect contact Geometry of construction: tubes, plates, and extended surfaces Heat transfer mechanisms: single phase or two phase flow Flow Arrangement: parallel flow, counter flow, or cross flow Fig3.2: Plate Heat Exchangers Plate and Shell Heat Exchanger A third type of heat exchanger is a plate and shell heat exchanger, which combines plate heat exchanger with shell and tube heat exchanger technologies. The heart of the heat exchanger contains a fully welded circular plate pack made by pressing and cutting round plates and welding them together. Nozzles carry flow in and out of the platepack (the 'Plate side'

4 flow path). The fully welded plate pack is assembled into an outer shell that creates a second flow path ( the 'Shell side'). Plate and shell technology offers high heat transfer, high pressure, high operating temperature, puling and close approach temperature. In particular, it does completely without gaskets, which provides security against leakage at high pressures and temperatures. Fig 4.1: Parallel flow heat exchanger Cross flow Heat Exchanger: In a cross-flow heat exchanger the direction of fluids are perpendicular to each other. Fig3.3: Plate and Shell Heat Exchanger Theory of Design and Analysis Design Considerations In designing heat exchangers, a number of factors that need to be considered are: 1. Resistance to heat transfer should be minimized 2.Contingencies should be anticipated via safety margins; for example, allowance for fouling during operation. 3. The equipment should be sturdy. 4. Cost and material requirements should be kept low. 5. Corrosion should be avoided. 6. Pumping cost should be kept low. 7. Space required should be kept low. 8. Required weight should be kept low. Classification of Heat Exchangers According To the Flow Direction a) Parallel flow b) Cross flow c) Counter flow Parallel flow Heat Exchanger In parallel flow heat exchanger, the two fluids flow in same direction and parallel to each other. Fig 4.2: Cross flow heat exchanger Counter Flow Heat Exchanger In a counter flow or countercurrent exchanger, as shown in Fig. the two fluids flow parallel to each other but in opposite directions within the core. The temperature variation of the two fluids in such an exchanger may be idealized as one-dimensional. The counter flow arrangement is thermodynamically superior to any other flow arrangement. It is the most efficient flow arrangement, producing the highest temperature change in each fluid compared to any other two-fluid flow arrangements for a given overall thermal conductance (UA), fluid flow rates (actually, fluid heat capacity rates), and fluid inlet temperatures. Moreover, the maximum temperature difference across the exchanger wall thickness (between the wall surfaces exposed on the hot and cold fluid sides) either at the hot-or cold-fluid end is the lowest, and produce minimum thermal stresses in the wall for an equivalent performance compared to any other flow arrangements. Classification of Heat Exchangers According To the construction Tubular heat exchangers Tubular heat exchangers are built of mainly of circular tubes there are some other geometry has also

5 been used in different applications. This design can be modified by length, diameter and physical arrangement. This type is used for liquid-to-liquid (phase changing like condensing or evaporation) heat transfer. Again this type is classified into shell and tube, double pipe and spiral tube heat exchangers. Double pipe heat exchanger The double pipe or the tube in tube type heat exchanger consists of one pipe placed concentrically inside another pipe having a greater diameter. The flow in this configuration can be of two types: parallel flow and counter-flow. It can be arranged in a lot of series and parallel configurations to meet the different heat transfer requirements. Double coil heat exchanger is widely used; knowledge about the heat transfer coefficient, pressure drop, and different flow patterns has been of much importance. The curvature in the tubes creates a secondary flow, which is normal to the primary axial direction of flow. This secondary flow increases the heat transfer between the wall and the flowing fluid. And they offer a greater heat transfer area within a small space, with greater heat transfer coefficients. The two basic boundary conditions that are faced in the applications are constant temperature and the constant heat flux of the wall stainless steel, copper, bronze, brass, titanium and various alloys. Generally, the outer shell is made of a durable, high strength metal, such as carbon steel or stainless steel. Inner tubes require an effective combination of durability, corrosion resistance and thermal conductivity. Regular materials used in their construction are copper, stainless steel, and copper/nickel alloy. Other metals are used in device fittings, end bonnets and heads. Heat Transfer Coefficient Convective heat transfer is the transfer of heat from one place to another by the movement of fluids due to the difference in density across a film of the surrounding fluid over the hot surface. Through this film heat transfer takes place by thermal conduction and as thermal conductivity of most fluids is low, the main resistance lies there. Heat transfer through the film can be enhanced by increasing the velocity of the fluid flowing over the surface which results in reduction in thickness of film. The equation for rate of heat transfer by convection under steady state is given by, Double pipe helical coil Close-up of double pipe coil Materials Used For Heat Exchangers A variety of materials are used in the design of tube heat exchangers, including carbon steel, Wall convection

6 The value of h depends upon the properties of fluid within the film region; hence it is called Heat Transfer Coefficient. It depends on the different properties of fluid, dimensions of the surface and velocity of the fluid flow (i.e. nature of flow). The overall heat transfer coefficient is the overall transfer rate of a series or parallel combination of convective and conductive walls. The overall Heat Transfer Coefficient is expressed in terms of thermal resistances of each fluid stream. The summation of individual resistances is the total thermal resistance and its inverse is the overall heat transfer coefficient, U. Modeling of double helical pipe heat exchanger Make sketch for helix Make helix by giving pitch and revolution Pitch: 40mm Revolution: 2 Where, U = overall heat transfer coefficient based on outside area of tube all A = area of tube wall h = convective heat transfer coefficient Rf = thermal resistance due to fouling Rw= thermal resistance due to wall conduction and suffixes O and I refer to the outer and inner tubes, respectively. Due to existence of the secondary flow, the heat transfer rates (& the fluid pressure drop) are greater in the case of a curved tube than in a corresponding straight tube at the same flow rate and the same temperature and same boundary conditions. SOLID WORKS Solid Works is mechanical design automation software that takes advantage of the familiar Microsoft Windows graphical user interface. It is an easy-to-learn tool which makes it possible for mechanical designers to quickly sketch ideas, experiment with features and dimensions, and produce models and detailed drawings. Use sweep feature command and generate inner pipe Use shell command and give thickness to pipe

7 Use sweep feature command and generate outer pipe. Fig : double helical pipe 3d model ANSYS ANSYS delivers innovative, dramatic simulation technology advances in every major Physics discipline, along with improvements in computing speed and enhancements to enabling technologies such as geometry handling, meshing and post-processing. These advancements alone represent a major step ahead on the path forward in Simulation Driven Product Development. But ANSYS has reached even further by delivering all this technology in an innovative simulation framework. Defining Material Properties. In this step, necessary thermal and mechanical material properties such as Young s modulus, Poisson s ratio, density, thermal expansion, convection, heat flow etc., are defined to the model. Generation of Mesh. In this step, the model is divided into finite pieces called nodes. Two nodes are connected by a line called Element. This network of elements together is called a Mesh. The boundary conditions are applied on the nodes and elements. CFD ANALYSIS Computational fluid dynamics (CFD) study of the system starts with the construction of desired geometry and mesh for modeling the dominion. Generally, geometry is simplified for the CFD studies. Meshing is the discretization of the domain into small volumes where the equations are solved by the help of iterative methods. Modeling starts with the describing of the boundary and initial conditions for the dominion and leads to modeling of the entire system. Finally, it is followed by the analysis of the results, conclusions and discussions. Model Convert the 3d model file to iges file and transfer it in ansys work bench. Mesh CFD analysis for parallel flow heat exchanger Name selection Assign the names for walls, inlets, outlets, and fluids, the different surfaces of the solid are named as per required inlets and outlets for inner and outer fluids. The outer wall is named as adiabatic wall.

8 Flow model Viscous model Select K-epsilon flow type Cell zone condition Select Fluid as water Select solid inner & outer pipe as steel Boundary conditions: Cold Inlet velocity: 2 m/s Hot inlet velocity: 1.8 m/s Cold inlet temperature: 303 k Hot inlet temperature: 353 k Cold outlet: pressure outlet Hot outlet: pressure outlet Hot & cold fluid: water Inner & outer Pipe material: steel Adiabatic wall Cold inlet Momentum, velocity: 2 m/s Thermal, Temperature: 303 k Outer pipe - Cold fluid Temperature Pressure Hot inlet: Momentum, velocity: 1.8 m/s Thermal, Temperature: 353 k

9 Velocity Helical double pipe: Temperature Inner pipe - Hot fluid Temperature Pressure Velocity: Velocity CFD Analysis for Counter Flow Heat Exchanger Every step will be same as parallel flow heat exchanger except name selection. Boundary conditions will be same as parallel flow heat exchanger. Name selection Assign the names for walls, inlets, outlets, and fluids, the different surfaces of the solid are named as per

10 required inlets and outlets for inner and outer fluids for counter flow heat exchanger. The outer wall is named as adiabatic wall. Outer pipe - Cold fluid Temperature Pressure

11 Velocity Helical double pipe Temperature Inner pipe - Hot fluid Temperature Pressure Pressure Velocity

12 Velocity Conclusions: Modeling and analysis of helical double pipe heat exchangers is done. Modeling of helical double pipe heat exchanger is done in solid works 2016 software using various commands. Model is transfer to ansys 14.5 work bench by converting it into iges file. CFD analysis is carried out in Ansys fluent for both parallel and counter flow of hot and cold fluid. Name selection is done as inlet, out let, fluid solid, walls are assign and mesh the helical double pipe heat exchanger. Water is used as hot and cold fluid and steel is used as material for both inner and outer pipe The boundary conditions are assign for parallel and counter flow type heat exchanger at inlet and outlet of pipes, outer wall of outer cold pipe is made as adiabatic. Temperatures, pressure and velocity of hot and cold fluid at outlet are found out as result of CFD analysis. Temperature, pressure, velocity counters all over the inner and outer pipe is shown. Hence the study of temperature,pressure and velocity because of parallel and counter flow in helical double pipe heat exchanger is done in this project References: 1. Experimental and CFD study of a single phase cone-shaped helical coiled heat exchanger: an empirical correlation. By Daniel Flórez-Orrego, ECOSJune 26-29, Helically Coiled Heat Exchangers by J.S.Jayakumar. 3. Numerical And Experimental Studies of a Double pipe Helical Heat Exchanger by Timothy John Rennie, Dept. of Bio-resource Engg. McGill University, Montreal August Experimental and CFD estimation of heat transfer in helically coiled heat exchangers by J.S. Jayakumar, S.M. Mahajani, J.C. Mandal, P.K. Vijayan, and Rohidas Bhoi, 2008, Chemical Engg Research and Design Heat Transfer Optimization of Shell-and-Tube Heat Exchanger through CFD Studies by Usman Ur Rehman, 2011, Chalmers University of Technology. 6. Structural and Thermal Analysis of Heat Exchanger with Tubes of Elliptical Shape by Nawras H. Mostafa Qusay R. Al-Hagag, IASJ, 2012,Vol-8 Issue Numerical analysis of forced convection heat transfer through helical channels Dr. K. E. Reby Roy, IJEST, July-2012 vol Minton P.E., Designing Spiral Tube Heat Exchangers, Chemical Engineering, May 1970, p Noble, M.A., Kamlani, J.S., and McKetta, J.J., Heat Transfer in Spiral Coils, Petroleum Engineer, April 1952, p Heat Transfer Analysis of Helical Coil Heat Exchanger with Circular and Square Coiled Pattern by Ashok B. Korane, P.S. Purandare, K.V. Mali, IJESR, June 2012, vol-2, issue- 6.