Heat Exchanger Design - Term Paper Example

Air-Air heat exchanger Name University November 2014 Abstract The designed heat exchanger uses a self contained process of heating simulation with an inbuilt controller function. In this engineering set up, the method that was utilised involved altering parameters under investigations and measuring their corresponding effects on the temperature changes, these were then compared with the set values so at to generate a control signal that could enable regulation of the electrical power supply to heaters. There is a need to therefore formulate ways and methods of ensuring such processes attain maximum utilization of the available resources at a controlled approach. Heating applications is the generation and transfer of heat as well as its regulation within set limits to avoid loss of this precious energy. Such settings would more often give a better combination of various parameters under investigations that could be unified to give the best result in any controlled engineering process. Table of Contents Abstract 2 Introduction 4 Methodology 4 Material selection 4 Design results and discussion 5 Discussion of Results 10 Modelling of interface duct 11 CFD analysis of duct 11 Results of modelling 12 Conclusion 13 References 14 Introduction Air-air heat exchangers are important especially during the cold season where the air outside the building or a vessel, aeroplane is cold as compared to the inside. Continued moisture may cause rust to the air craft or cause mould to the building. When the incoming air is very cold, there is need to use air-ait heat exchanger to avoid ice forming on the machine or building involved. The heat exchanger used will solve the problem within a certain period depending on their efficiency. Air-air heat exchanger will be efficient if it is designed well. Methodology The product to be designed is Air-air heat exchanger and should fit in an envelope of 0.20m and have a clearance of 0.015m around it. It should not weigh more than 25kg. Material selection In our design radiation of heat important but it is difficult to quantify its effects thus we choose aluminum 6000 series of emissivity of 0.05. It is known that hot air moves upwards as compared to cold air thus the materials used will affect the airflow in the heat exchanger. The number of fins used in our design can be noted to be nine of aluminum that had thickness of 15mm and gap between them of 7.5mm. These materials that were used are good for heat dissipation especial when forced convection is necessary. In this case Plate-fins are helpful in ensuring that airflow is over the plates. It should be noted that Pin fins are essential when the turbulence of airflow is not known. This is the reason why pin fins have been chosen to avoid airflow diversion from the heat exchanger by using plate-fins. The measurement of pin fins was cut from the aluminium sheet that was available and was arranged on the top and right sides as shown in the diagram below. Figure 1: Plan view of heat exchanger Design results and discussion From the results of design radiation and convection coefficients are 0.001088W/cm2 and 0.000262W/cm2 respectively. It implies that more heat lost through radiation than convection in heat exchanger is radiation coefficient is higher than convection coefficient. Heat Transfer Rate from Rectangular Fin to Surrounding Air h k h h extrusion length width (m) 0.2 0.2 0.2 0.2 0.2 0.2 thickness thickness (m) 0.002 0.002 0.002 0.002 0.002 0.002 protrusion out length (m) 0.05 0.075 0.1 0.125 0.175 0.2 # fins 17 17 17 17 17 17 k (W/m/K) 202 212 222 232 242 249 h (W/m2/K) 0.004 0.004 0.004 0.004 0.004 0.004 ambient air T_infinity (deg K) 300 300 300 300 300 300 width (m) T_base (deg K) 380 380 380 380 380 380 Computed Values theta_b 80 80 80 80 80 80 perimeter (P) 0.404 0.404 0.404 0.404 0.404 0.404 area (Ac) 0.000 0.000 0.000 0.000 0.000 0.000 M 0.914 0.937 0.958 0.980 1.001 1.015 m^2 0.020 0.019 0.018 0.017 0.017 0.016 m 0.141 0.138 0.135 0.132 0.129 0.127 Results             Heat Transfer Rate per Fin (mW) 7 10 13 23 26 26 target rate 200mW Heat Transfer Rate for All Fins (mW) 112 167 222 387 442 442 Selected                 Heatexchanger Details           Fin Depth 20.00 mm Heat exchanger Calculator       Fin Height 20.00 mm No. Fins 17   Length 200.00 mm Width 10.00 Mm       Thermal Ratings       Temperature (maximum) Per Fin 15.7433 deg K / Watt   Heat exchanger 474.00 deg K H/S Body 138.1487 deg K / W     Ambient 429.00 deg K Total 5.2611 deg K / Watt   Emissivity 0.05                         Surface Treatment Emissivity (typical) Coefficients:             Min Convection 0.001088W/sq cm 0.0070 W/sq in Polished aluminium 0.05 Radiation 0.000262W/sq cm 0.0017 W/sq in           Determine the transfer characteristics of in-line tube banks using =13.297 Where subscript f is the film temperature, C and n are constants C and n, is Reynolds number based on the maximum velocity occurring in the tube bank, i.e., the velocity through the minimum-flow area and depend on = 5 The empirical friction factor f’ is given by: = 0.55635x (749.15)-0.15 = 0.9269 The graph below shows that when the fin length increases, the heat transfer rate increases but which means long fins will have low thermal resistance due to conduction. The same case applies to short fins where thermal resistance increases and has reduced the convection heat transfer. The pattern of the plotted points on the graph slopes from right to left of the scatter plot suggesting a positive relationship between the variables. This kind of association simply implies that as the length increases heat transfer rate changes slightly. This kind of findings simply goes with the general expectation. The finding through the scatter diagram is further reinforced by the value of the correlation coefficient between the variables under consideration. The positive sign of the coefficient indicates a positive relationship while the very low absolute value of the coefficient, simply implies minimum relationship in the figure below. The velocity profile is thus constrained to a no-growth condition and fully develops only after traversing airstream. Theoretically the airstream heat increase recorded would be linear. The transverse velocity profile, mentioned above, also helps in understanding the development of the boundary layer. The fully developed air flow would either be laminar or turbulent and would be validated by the Reynolds number calculated for the velocity at a particular cross section under review. Heat Exchanger Dimentions Set the values in the green cells and find your results Length 0.65m Calculated Properties Results Width 0.45m Ab 0.002 Rbase 0.040161 Depth 0.20m Ac 0.0002 Rconvection with fins 2.905217 At 0.0088 Rconvection no fins 12.5736 Fin Dimentions Af 0.0006 q no fins 6.342278 Height 0.02 Ab,0 -0.0014 q with fins 27.1612 Width 0.02 Ap 0.0004 Fin Array Effectivness 4.282562 Depth 0.01 Lc 0.03 P 0.06 Number of fins 12 nf 0.98587 no 0.983623 Properties m 6.921753 q goal 200 k 294 Tb 474 Tair 429 hair 39.76585 The convection coefficient for the front rectangular-fins was found at the middle position of the fins, where as the rear fins’ convection coefficient was found at the rear point of the base plate. Presents the results: h front plate-fins h back plate-fins 20.673 9.52 q fat fins(W) q thin fins(W) q front-plate fins (W) q back plate-fins (W) q total (W) 13.4 10.16 3.5 1.61 28.67 Rtotal K/W) 0.87 Discussion of Results There was heat transfer coefficient of 40W/m2.K and at 0.2kg/s, while its corresponding cold having an efficiency of 105%. Calculated power conducted in this case is 10.8835 watts while power through convectional was 2.229watts. The variation of fin length gave different heat Transfer Rate values which calculation of the overall efficiency could be done together with performance of energy balance across the heat exchanger. In this case it is clear that there was absorption of heat energy from the environment.. Whereby the airflows to the system from different sides of the system and then meets each goes down and hot air goes up. This is opposed to parallel heat exchanger systems in which the air would enter the system on one end and then flow parallel to each other for exchange to take place. As these exchanges positions because of cases in weight, heat would be transferred from one lower side where there are higher temperatures to the one with lower temperatures. The eventual temperature at the outlet would be different from that recorded from the initial setting. It is from these temperature differences that their mean and differences calculates efficiency of the system at various variations of flow rates. Once the system is on, temperature changes were recorded alongside their flow rates. Modelling of interface duct CFD analysis of duct BONDARY CONDITIONS Utilising the boundary conditions that are constrained by the gaseous phase temperature, heat flux is assigned the temperature value. This gives a way forward to solve the two forms of energy equations of the structural element, with the equation being dependent on the thermal width of the material: 𝜌𝑠𝐶𝑝𝑠𝑉 𝑠 𝑑𝑇/𝑑𝑡 = 𝐴𝑠𝑞"𝑠 ………….. [Thermally Thin Material- (that is Steel)] The boundary conditions are defined by the fire and are given by: The methodology possess major physical flaws with the definition of comprising of two components, which are radioactive and convective. The former component entails all the sources of radiations giving a boundary condition of: Where is the convective contribution is the part describing the phenomenon of the surface re-radiation and the term constitutes all the possible radioactive inputs. These radioactive inputs can be due to the hot gases. Results of modelling The modeling below shows heat exchanger and distribution of heat when cooling the incoming air. The effect of inlet air induces internal flows within the boundary layer exchanger with its induced pressure load on surface and inside through the openings. There is substantial variation in temperature of air flows. The internal pressure induced by inlet air may take a substantial proportion of design specific for design. There is occurrence of buildup pressures in addition to the momentum of air pressure where the opening is considerably wide. Mean pressure data is the most popular method used in the prediction of the outlet temperature. However, the method is associated with some discrepancies when measurements are made. The figure below shows simulated results of velocities of air inlet and outlet where temperature for outlet air is Conclusion The designed exchanger was a simple model that used flat pin fins to allow convection from a flat plate to the air flow. The convection coefficient was calculated and it was discovered that was rear fins’ convection coefficient was at the base while middle fins had in the middle. The design was done successfully to understand the methods of flow measurement and the concepts of flow process that happens inside the heat exchanger. Determining the velocities along a particular cross section by the measurement of heat transfer and the application of the Bernoulli’s principle in determining the velocities were done successfully. The graphical representation of the velocities at various positions over the length helped understand the basis of how a boundary layer is formed and how it affects the flow process. References Incropera, F.P. & DeWitt, D.P., 2002. Fundamentals of Heat and Mass Transfer, 4th Edition, New York: John Wiley & Sons Kragh J., Rose, J, Nielsen, T.R. & Svendsen, S., 2006.New counter flow heat exchanger designed for ventilation systems in cold climates Kragh,J., Rose,J. & S. Svendsen,S. 2005. Mechanical ventilation system with heat recovery in cold climates, in: Proceeding of the Seventh Nordic Symposium on Building Physics in Nordic Countries, vol. 2, Technical University of Denmark. LBNL., 2003. Therm ver. 5.2, Finite Element Simulator, Lawrence BerkleyNational Laboratory, USA,. Read More