With the continuous development of industrialization, compressed air is widely used in the mechanical field as a power source. In some cases, higher requirements are placed on the pressure and dryness of compressed air. The dryer for drying and filtering high-pressure air can be classified into a non-thermal regeneration, a micro-thermal regeneration, and a heating regeneration adsorption dryer according to the regeneration principle of the adsorbent. The workflow of the high-pressure micro-heat regeneration adsorption drying filter is mainly divided into two processes of adsorption drying and regeneration analysis. Therefore, the analytical compressed air should be decompressed from the dried high-pressure product gas, and during the decompression process, the compressed air temperature will drop sharply. For example, when the pressure is reduced from 10 to 40 MPa to 0.5 to 1.5 MPa, the temperature may be from 40 ° C to below 0 ° C, is obviously not conducive to the analysis of the adsorbent, so the compressed air after decompression needs to be heated.
Aiming at this problem, this paper calculates the relevant parameters of the traditional compressed air heater, and analyzes the internal temperature field and flow field of the heater with the simulation analysis software Fluent; the structure of the heater is optimized according to the simulation analysis results. A cross baffle structure is proposed.
1 Compressed air heater structure As shown, the structure of the compressed air heater is mainly composed of an electric heating tube and a heater box. The stainless steel electric heating tube has an electric heating wire inside, and the void portion is filled with crystalline magnesium oxide powder. The crystalline magnesium oxide powder has good thermal conductivity and electrical insulation. When a high-temperature electric resistance wire is supplied with electric current, the generated heat is transmitted to the metal pipe through the crystalline magnesium oxide powder, and the metal pipe heats the compressed air by convection and radiation heat exchange. . The use of stainless steel tubes reduces the oxidation of air at high temperatures.
Zhang Siping, et al.: CFD-based compressed air heater improved design optimization and improved internal structure of compressed air heater 2 Theoretical design and calculation The convective heat transfer of compressed air in the heater can be divided into natural convection and forced convection, forced convection The heat transfer intensity is greater than the natural convection, so forced convection heat transfer plays an important role in improving the heat transfer efficiency of the heater.
2.1 Known conditions of calculation Through the analysis of a micro-heat regeneration adsorption dryer, the temperature of the high-pressure air after passing through the pressure-reducing valve is approximately equal to °C, and the demand of the drying tower for analyzing the compressed air is: the pressure is about 0.5 MPa, the temperature is about For 100C, its flow rate under standard conditions is about 10m3/h. 2.2 Design calculation of various parameters of the heater By calculating the total heat required for heating the air, the average temperature of heating and the heat transfer coefficient, the traditional staggered structure is calculated. The number of heating tubes in the heater.
2.2.1 The total heat required for heating is lower after the decompressed compressed air. After heating, the temperature of the compressed air rises and the pressure increases. According to the conservation of energy, the temperature is caused by the engineering design calculation. Pressure changes.
The whole heat transfer process is variable temperature heat transfer. The temperature difference between the wall surface of the heating pipe and the inlet of the compressed air and the temperature difference of the outlet are large. The surface temperature of the heating pipe wall is set to 120C, the inlet temperature h of the compressed air is 0C, and the outlet temperature t2 is 100C. Then the logarithmic mean temperature difference is 120C; A. is the heating pipe and the compressed air at the outlet of the heating pipe. Therefore, the logarithmic mean temperature difference Atm enters the heater. The compressed air flow rate and density are: 1013 MPa; 9 is the air at standard atmospheric pressure. Flow rate, 10m3/h; P1 is the compressed air pressure in the heating pipe, 0.5MPa; T1 is the average temperature of the compressed air in the heating pipe, 333K; 1 is the molar weight of air, 28.97 kg/ml; R is the gas constant, 8314 J / (mo b K).
Therefore, the total heat transfer required to heat compressed air at a flow rate of 0.5 MPa compressed air in the heating tube is the port temperature, 273 K; T2 is the outlet temperature, 373 K. Check the physical properties of the compressed air: the dynamic viscosity of the air does not substantially change with the pressure And the change, so the kinematic viscosity is 2.2. The relevant parameters of the heater assume that the selected heating tube diameter is 20mm, and the heating tube adopts the staggered structure, the tube spacing A=X2=30mm, the number of staggered rows is above 16 rows, the heater The flow area A at the narrowest point is 100mmx30mm, and the flow rate of the compressed air at the narrowest point is the Reynolds coefficient of the compressed tube when the compressed air flows in the heater. Therefore, the Reynolds coefficient Re=976. According to the above conditions, The Siel coefficient is the flow field vector in the heat exchanger coefficient of the compressed air temperature profile heater in the ship science technology heater. 2.2 Simulation results and analysis of the baffle structure In the structure of the heater model, the long pipe is shortened by the baffle plate and the height is increased. Through numerical simulation, the temperature of the compressed air outlet can be seen. It is 365K, but the number of heating pipes in the heater is 20, which is reduced by 1/3 compared with the conventional staggered structure. Therefore, the heater structure improves the heating efficiency. In the place indicated by the black arrow in the internal flow field vector diagram of the heater, a compressed air retention zone is formed, in which the compressed air flow rate is low, and the thermal conductivity of the compressed air is extremely low, so the formation of the region beside the heating pipe Not conducive to the heating of compressed air. It can be seen from the analysis that changing the position of the compressed air retention zone away from the heating pipe or strengthening the air flow in the region shows the correctness of the simulation model to a certain extent compared with the simulation results. It can be seen that the gas flow rate in the middle of the heater is relatively high, and the main flow of the compressed air in the heater is distributed inside the heating pipe, and the flow rate of the compressed air in contact with the wall surface of the heater casing outside the heating pipe is low.
The temperature profile of the compressed air in the heater. Therefore, the heat transfer coefficient a can be known from the basic equation of heat transfer. Therefore, the equations (10) and (11) are available, n=31.3. The simulation uses Fluent software to numerically simulate the internal flow field and temperature field of the traditional staggered structure heater, and compared with the theoretical calculation results, the results show very high performance. Therefore, according to the flow characteristics of the internal flow field obtained by the simulation result, the heater structure is improved, and the flow path in the heater is changed, which not only makes the structure of the heater more compact, but also improves the heating efficiency.
3.1 Calculation model and its boundary conditions The heater adopts a structure in which the heating pipes are staggered. One end of the heater is the inlet of compressed air, and the other end is the outlet of hot compressed air. The model adopts the steady state model of the pressure solver, and chooses the hidden Solving format; selecting the energy calculation equation, the turbulence model adopts the standard k-mould type; the entrance of the simulation model uses the velocity inlet boundary condition; the flow field is set to compressed air, and the density is modified to the actual density value at 0.5 MPa. The other items were kept at the original values; the wall temperature of the heating tube was set to 393 K, and no heat exchange occurred between the wall and the outside. The above-mentioned heater simulation calculation model ignores the change in pressure and density due to the increase in the temperature of the compressed air, and similarly ignores the change in the thermal conductivity caused by the change in density and temperature.
3.2 Simulation results and analysis The structure of the first heater simulation model is a traditional heating tube staggered structure; the second model is to reduce the length of the heater, the baffle is designed, the height of the heater is increased, and the length is shortened. Numerical simulation calculations show the distribution of internal temperature field and flow field. Compared with the traditional structure of the heater, the number of heating tubes is reduced, indicating that the heating efficiency is improved. The third structure is the second heating. On the basis of the internal flow field, the baffle of the heater was improved and transformed into a cross baffle. The numerical calculation found that the heat transfer efficiency was further improved, and the design basis before the improvement was verified.
3.2.1 Simulation results and analysis of the traditional staggered heating structure In the middle, the heater height is 50mm, the heating pipe diameter is 20mm, the heating pipe is staggered according to the design, a total of 30 heating pipes, the compressed air temperature at the inlet is about 273K After heating, the outlet compressed air temperature reaches 373K, theoretical design results Zhang Siping, etc.: CFD-based compressed air heater improves the flow field vector in the design heater. 2.3 The simulation results and analysis of the cross baffle structure are the improved cross baffle heater structure model. The number of heating pipes is 20, and the boundary conditions of the compressed air inlet remain unchanged, and the compressed air outlet temperature reaches 375K, the outlet temperature of the baffle structure is about 10 higher; from the flow field distribution diagram inside the heating pipe, the flow velocity on both sides of the heating pipe is higher, which enhances the forced convection heat transfer effect of the compressed air to a certain extent; The compressed air forms a vortex near the cross plate, which is far away from the heating pipe. Therefore, it is feasible to change the flow field structure in the heat exchanger by adjusting the structure and improve the heat exchange efficiency of the heater.
Compressed air temperature profile in the heater Flow field vector in the heater The design method of a compressed air heater is given in this paper. The internal structure of the heater is optimized by CFD technology, and the following conclusions are obtained: Through the calculation analysis and simulation analysis of the traditional staggered heater, the consistency of the results is relatively high, indicating that the internal structure of the heater can be improved by simulation calculation.
2) Through the simulation analysis of the internal flow field of the heater, it is found that the traditional misaligned heater has a higher intermediate gas flow rate, while the flow velocity on both sides is lower, the internal flow field is single, and the forced convection heat transfer effect is poor, resulting in heating. The efficiency is low; there is an air retention zone inside the baffle structure heater, and the presence of these regions is also not conducive to the improvement of heating efficiency; the gas flow around the heating pipe of the cross baffle heater is complicated, and the forced convection effect is improved. The simulation results also show that the heating efficiency of this type of heater is further improved.
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