Air-Air Heat Exchanger - Term Paper Example

Air-Air heat exchanger Name University November 2014 Abstract The design ascertains how heat exchanger could be coordinated under various variations of its heating parameters to give a more reliable and controllable functioning of equipment. Model geometry is created, meshed, calculated, and post-processed using CFD. It is from this understanding that this report is based in an attempt of communicating these findings. Various methods of heat reduction were available to the company including thermal radiation and convection by cold air in heat exchanger. Aluminium was used because of its thermal conductivity of kAL = 202 – 249 W/mK and its relatively low density ρAL = 2700 kg/m3 that is a desirable for a low weight solution. Therefore, a slight change in parameters like cold airflow rates could bring an effect on the entire processing unit. How these changes could be coordinated to give a more desired aftermath was integral in this design. Of these, three types of heat exchangers were used for the entire Simulational design; these were shell and tube, concentric tube, and the plate systems of exchangers. This kind of design was important to help compare overall efficiency of these systems at different flow rates when both cold and hot flow rates are varied at different inlet and outlet temperatures. Based on results obtained each from the three systems, calculation of efficiency from these various variations were important in deducing the most effective rate at which the exchanger would give the desired efficiency and thus function at the optimum. Energy balance across the heat exchanger was examined. Obtained results show that the efficiency of the machines was varied with an effect of changing both flow rates. Various temperatures were indicated alongside these variations; however, some setting did not indicate consistent variations when the flow rates were varied. The first Simulation showed a more close prediction to what was expected, since efficiency remained within a given range. Table of Contents Abstract 2 Introduction 4 Methodology 4 Material selection 4 Design results and discussion 5 Discussion of Results 9 Modelling of interface duct 10 Similarly, the heat gain rate of the cold air is 13 Understanding of results from the CFD analysis 15 Conclusion 19 References 20 Introduction Heat exchangers form a critical piece of aircraft engine and are particularly important for those that rely on aircraft makers. A vast array of heat exchange equipment tailored to meet specific needs exists. They rely on transfer of heat energy from one fluid stream to another, use different configurations to separate the two streams and obtain specific flow patterns, and use different kinds of materials for construction most commonly metals and alloys. Broadly, classification of heat exchangers depends upon the flow arrangement and type of construction (Incropera and Dewitt, 2002; Green and Perry, 2008). 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. 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 The materials that will be used will be Aluminum 6000 series in accordance with MIL-HDBK-5J. These materials that were used are good for heat dissipation especial when forced convection is necessary. Aluminium was used because of its thermal conductivity of kAL = 202 – 249 W/mK and its relatively low density ρAL = 2700 kg/m3 that is a desirable for a low weight solution. The increase of the thickness or diameter will increase the surface area leading to improvement of heat exchange. Thermal radiation is another way there is heat transfer which depends on the on the emission coefficient of the aluminum that is low emission coefficient as compared to others. 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. Selected Hot side Calculations Hot air Ram side Calculations Cold air Flow Characteristics Flow Characteristics Inlet Temperature Tin = 200.00 C Inlet Temperature Tin = 20.00 C Outlet Temperature Tout = 155.00 C Outlet Temperature Tout = 22.00 C Mean Temperature Tmean = 450.65 K Mean Temperature Tmean = 294.15 K air k = 0.04 W/mK Air k = 0.03 W/mK Cp = 1023.35 J/(kg K) Cp = 1006.90 J/(kg K)  = 0.000025 Pa s  = 0.000018 Pa s  = 0.79 kg/m3  = 1.18 kg/m3 Pr = 0.68 Pr = 0.71 Mass Flow Rate m = 0.200 kg/s Mass Flow Rate m = 0.100 kg/s Capacity mCp = 204.7 W/K Capacity mCp = 100.7 W/K Heat Transfer Q = 9210 W Heat Transfer Q = 201 W Internal Flow External Flow Inside Diameter di = 0.2000 m Inside Diameter di = 0.0160 m Outside Diameter do = 0.1900 m Outside Diameter do = 0.0190 m Flow Area Ac = 0.0314 m2 Flow Area Ac = N/A m2 Hydraulic Diameter Dh = 0.2000 m Hydraulic Diameter Dh = N/A m Heat Transfer Surface Area As = 0.4084 m2 Heat Transfer Surface Area As = 0.0388 m2 allumium Conductivity kw = 300.0 W/mK Pipe Conductivity kw = 300.0 W/mK Fluid Velocity V = 0.804 m/s   Calculations Calculations Reynolds Number Re = 5086.34 Reynolds Number Re = N/A Loss Coefficient K = N/A Loss Coefficient K = N/A Friction Factor f = 0.00960 Friction Factor f = N/A Nusselt Number Nu = 15.38 Nusselt Number Nu = N/A Heat Transfer Coefficient h = 2.88 W/m2K Heat Transfer Coefficient h = 5000.00 W/m2K Pressure Drop P = 0.03 Pa Pressure Drop P = N/A Pa Heat Exchanger Parameters Log Mean Temperature Difference TLMTD = 155.51 C Mimimum Fluid Cmin = 101 W/K Cr=Cmin/Cmax Cr = 0.49 Q max Qmax = 18124 W Overall Heat Transfer Coefficient UA = 12 W/K Effectiveness  = 0.11 Number of Transfer Units NTU = 0.12 Required Transfer Coefficient UA = 12 W/K Heat Transfer Q = 1934 W Length L = 0.6500 m Number of Tubes N = 10.0 air frontal U Reynolds Re j f P drop 0,3 330 0,0420 0,1100 0,32 0,5 600 0,0270 0,0730 0,65 0,7 790 0,0230 0,0630 1,15 1,1 1300 0,0170 0,0460 2,29 1,5 1700 0,0140 0,0420 4,02 2,5 2900 0,0120 0,0330 9,67 3,7 4300 0,0094 0,0270 19,02 4,5 5200 0,0090 0,0240 26,53 5,4 6200 0,0084 0,0220 36,67 6,2 7000 0,0081 0,0210 47,32 RESULTS 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. 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 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: Continuity equation: Momentum equation: Energy equation = General transport equation (for scalars): The mesh has been built to have two boundaries around the entire outer edge. This boundary is split into inlet and outlet regions.The boundaries of the model were created as shown in the diagram below Figure 2: mesh Heat is a form of energy that flows from a hotter body to a colder one due to the temperature difference and may take any one or a combination of three modes namely, conduction, convection, and radiation. In heat exchangers that handle heat transfer across two airs, radiation rarely plays any role. The rate of heat transfer follows Newton’s law of cooling in that the rate of transfer of energy is directly proportional to the temperature difference. To this basic principle is added the concept of resistance as for electric energy transfer phenomenon which makes the heat flow rate directly proportional to the driving force (temperature difference) and indirectly proportional to the resistance to heat flow. The resistance to heat flow is a factor of the attributes of the material as well as the area available for heat transfer. The rate of heat flow at any point (kW Heat tra /m2 of transfer surface) depends on: nsfer coefficient (U), itself a function of the properties of the airs involved, air velocity, materials of construction, geometry and cleanliness of the exchanger Temperature difference between hot and cold airs Total heat transferred (Q) depends on: Heat transfer surface area (A) Heat transfer coefficient Average temperature difference between the airs, strictly the log mean (DTLM) The total heat transferred Q = UADTLM The heat transfer rate is expressed as: Transfer Rate = Transfer Coefficient x Transfer Area x Temperature Difference Given a situation where the two airs and their temperatures are given the problem of heat exchange equipment becomes one of increasing the area available for heat transfer. However, Incropera and Dewitt (2002) point to another factor that affects heat transfer across a surface with air on either side. This depends on the flow characteristics of the air on either side of the heat transfer surface. Close to the surface, a thin film forms where the flow is laminar and its thickness depends on the rate of flow, the viscosity of the air, the turbulence of the flow materials of construction of the heat exchanger, and the cleanliness of the transfer surfaces. In a given heat exchanger, operating under fixed conditions, it is not necessary to undertake calculations that require incorporation of the different factors that affect overall heat transfer and heat transfer rate mentioned above. By monitoring the flow rates and the inlet and outlet temperatures of both hot and cold airs it is possible to calculate the overall heat transfer coefficient using only the specific heat capacity of the two airs. The heat loss rate from the hot air is Qhot = Vhot x Cphot x (Tinlet – Toutlet) Similarly, the heat gain rate of the cold air is Qcold = Vcold x Cpcold x (Toutlet – Tinlet) Where, Q is the heat loss or gain rate, V the mass flow rate of the relevant air, Cp the specific heat of the airs and T the temperature. It is important to note that the density and specific heat of airs change with temperature and it is important to adjust the mass flow rate and the specific heat to the actual temperatures obtaining in the simulation. One may take additional readings of the temperature at the mid-point of a concentric tube heat exchanger but this is very difficult to achieve in other configurations and it is normal to take an average of the inlet and outlet temperatures for adjustment of the flow rate and the specific heat of the airs. Because heat exchangers operate in an open environment and particularly in the case of laboratory heat exchangers for student demonstration the equipment is not insulated, it is expected that some heat from the system will either be lost to the environment, or gained from it affecting the overall efficiency of the heat exchanger. The efficiency of the heat exchanger is thus a ratio of the heat gained by the cold air to the heat lost by the hot air. The overall heat transfer coefficient of the equipment can also be calculated from the equation For this we need to calculate the heat exchange area and the log mean temperature difference (LMTD), which is calculated as using the formula Where, ΔTa is the temperature difference between the two airs at the inlet of the hot air end and  ΔTb is the temperature difference at the exit end (Kern, 1950; Green and Perry, 2008). There was heat transfer rate of 40.oK/W and at 0.5m/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. One major observation from these data findings is that as the flow rate for cold air was maintained constant, variation of lengths rate from a lower value to a more significant value gave an increased thermal efficiency. The other observation is that some extra heat could be conducted materials, thus variation of these gave different values which determinations of the overall efficiency is possible. 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. The wide fluctuations and waviness of the graph could allude to the turbulence, or a phase of transition to turbulence flow property inside the pipe. Further to that, the fact that the friction factor of the pipe causes a marginal energy loss and thus a reduced pressure head difference could I turn be a limiting factor to properly evaluate the data and understand the flow process. The Ram side- will let in air of 20 and mass flow rate: 0.1kg/s. Maximum Pressure drop allowed:12KPathe ram side is the critical side for the performance of the heat exchanger. Hot Air side-Inlet Temperature: 200 and Inlet mass flow rate: 155. The number of layers and layer height for the hot side should be kept identical to ram side. For this heat Exchanger design, assume an overall heat transfer coefficient of 40W/m2.K. Understanding of results from the CFD analysis Computational Air Dynamics is mathematical modelling tool in a computer software which uses theory and some input of heat transfer and air flow dynamics. It gives primary technique for large tabulation of data that carries the air flow system and require direct resolutions so as to represent a flow process that has the desired accuracy. It uses two-air model, it is performed to get a flow pattern and the gas flow. The modelling has the ability to reduce computational demand with the overall speed of simulation process being improved. It modelling does not require the use of the averaged parameters and this makes it possible for a transient solution being obtained easily. With the availability of an inaccurate prediction of an incomplete burning levels thus impacts the calculations derived from radioactive heat transfer and burning rates which are estimated by human tenability’s. High quality which comes in with quantified uncertainty and relatively low temperatures provides measurements of heat flow from the interior of the under ventilated environment that are needed for guiding the development and also for validation of improved fire fields models. 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. In the simulation on the heat exchanger, the system gave four sets of values based on different flow rates for cold air rates at a constant hot air flow rate. In the first result where the hot air flow rate set at 0.1kg/sec, while its corresponding cold air flow rate of 0.108 kg/sec the efficiency obtained was 108%. Calculated power absorbance in this case is 7962 watts while power emitted is 7886 watts. Within the same simulated set up, variation of flow rates gave different temperature values for inlet and outlet upon which calculation of the overall efficiency could be done together with performance of energy balance across the heat exchanger system One major observation from these data findings is that as the flow rate for hot air was maintained constant, variation of cold air flow rate from a lower value to a more significant value gave an increased thermal efficiency. The figure below shows the distribution of temperature within the simulator The Figure 3L Distribution of temperature The figure above shows how air is cooled with within the exchanger. The air temperature is observed to change in channel as well the distribution of the temperatures changes the same way. The temperature is lower towards the outlet of the exchange. Figure 4: Air distribution during cooling Figure 5: Temperature distribution among fins The figure below shows simulated results of velocities of air inlet and outlet where temperature for outlet air is Figure 6: Air velocity Conclusion Results obtained from the Simulation had one major observation in that as the flow rate for hot air was maintained constant, variation of cold air flow rate from a lower value to a more significant value gave an increased thermal efficiency. From Simulation arrangement in part two, where both the cold and hot flow rates were readjusted linearly, various calculated efficiency values indicates that at high value of hot flow, the efficiency is higher than 100%, however, as the hot flow rate is reduced with an increasing cold flow rate the efficiency remains relatively constant within a certain marginal value. References Read More