Numerical simulation and optimization of heat transfer in a heat exchanger of a biomass hot air furnace

Biomass hot air furnace as a kind of energy-saving, environmentally friendly heating and warming device, has been more and more applications, can be used as a heating device in the winter, but also for grain crops and other drying to provide a heat source, but also for greenhouse greenhouse insulation and so on. Biomass hot air furnace mainly includes blower, combustion device, heat exchanger, etc., in which the heat exchanger is a part of the heat of the hot fluid to the cold fluid transfer device, biomass hot air furnace economy, reliability and usability is largely affected by the structure of the heat exchanger.

　　Due to the complexity of the heat exchanger structure, affecting the heat transfer efficiency of many factors, if you rely only on the test to optimize the structure of the heat exchanger in order to maximize its heat transfer efficiency, it will be a cumbersome and lengthy process. With the development of computer technology and the continuous improvement of computational fluid dynamics (CFD) knowledge, the calculation speed, stability and accuracy of CFD software have reached a reliable level. Therefore, CFD analysis of a biomass hot air furnace is carried out to derive the temperature and velocity fields of the internal airflow, which is then evaluated and optimized to improve the heat transfer efficiency, and finally the rationality of the structure is verified through tests.

01

Simulation Model

　　1.1 Establishment of physical model

The heat exchanger of this hot air furnace is an inter-wall heat exchanger, where two fluids with different temperatures flow in a space separated by a non-insulated wall surface, and heat exchange between the two fluids is realized through wall heat transfer and convection of the fluids on the surface of the heat-conducting wall. There are generally two types of heat exchangers, shell and tube and casing, and the shell and tube heat exchanger is simulated here.

　　The physical model is the entity basis for subsequent simulation, a reasonable physical model can save a lot of unnecessary trouble for subsequent meshing and analysis and calculation. In the three-dimensional software Catia to establish the physical model of the heat exchanger as shown in 1, the main parameters of the heat exchanger as shown in Table 1.

　　Below the heat exchanger is the combustion chamber, straw and other biomass fuels below the combustion, resulting in a large amount of heat accompanied by smoke flow into the heat exchanger tube (tube flow), heat exchanger above (i.e., the smoke outlet) equipped with induced draft fan to make biomass fuels burn more vigorously, generating more heat. Clean cold air from the heat exchanger below the square inlet into the (shell process fluid), through the wall of the thermal conductivity and cold air in the heat exchanger tube wall surface convection to obtain heat, from the heat exchanger above the circular outlet into the required hot air.

　　1.2 Determination of calculation model

　　Fluid flow is governed by physical conservation laws, the basic conservation laws include: the law of conservation of mass, the law of conservation of momentum, the law of conservation of energy. If the flow contains a mixture or interaction of different components (group elements), the system will also obey the law of conservation of components. If the flow is in a turbulent state, the system also obeys additional turbulent transport equations [1].

　　Mathematical modeling of heat transfer in heat exchangers is done with three-dimensional incompressible equations for conservation of mass, conservation of momentum, conservation of energy and thermal conductivity, and turbulence.Fluent provides a variety of turbulence models; however, no single model is universal for all problems. The standard k-ω model was chosen by considering the compressibility of the fluid, the accuracy of the computation, the CPU capacity of the computer, the time spent, etc. The standard k-ω model predicts the rate of propagation of the free shear flow, like wake, mixing flow, flat plate rapping, aid rapping, and radial jet, and thus can be applied to both wall-bound and free shear flows.

　　1.3 Model Processing and Meshing

　　1.3.1 Model Processing

The arrangement of heat exchanger tubes is designed to be symmetrical in order to facilitate the analysis. The 3D physical model is imported into the Geometry unit module of Workbench and the symmetry plane is created. Next, the fluid domain of the heat exchanger is extracted, and the extracted air and flue gas fluid domains are shown in 2 and 3, respectively.

　　1.3.2 Gridding

　　Mesh delineation is an extremely important part in the whole numerical simulation, the mesh is good or bad, which directly affects the accuracy of the solution, if the mesh is very poor or even can not be solved. In this model, it is necessary to set up the Mesh interface, the two Interface nodes should be as consistent as possible, and the mesh size should be as close as possible, otherwise, it cannot be well coupled when generating the Mesh interface, which leads to unsolvable. After several tests and comparisons, the finalized mesh size is set as shown in 4. Both the flue gas fluid domain and the air fluid domain are set with 3 boundary layers, the first layer is 1mm, and the growth factor is 1.2.

02

Initialization and Boundary Condition Setting

　　Since the temperature and pressure in the hot blast furnace are high, the heat exchanger pipe material is 20# steel, and Q235 steel is used elsewhere to save cost, and the physical parameters of these two materials are shown in Table 2. The physical parameters of flue gas and air at each temperature are shown in Table 3 and Table 4 respectively.

　　Boundary conditions to be set: ① two velocity inlet boundary conditions, including fluid inlet velocity, pressure, temperature, turbulence intensity, hydraulic diameter; ② two pressure outlet boundary conditions, including the relative pressure at the exit, turbulence intensity, hydraulic diameter; ③ wall boundary conditions, mainly wall temperature.

 
03
 

Simulation results and analysis

　　3.1 Analysis of tube flow field

　　The study of heat exchanger tube is one of the focuses of heat exchanger structure research, reasonable heat exchanger tube structure can largely improve the thermal efficiency of heat exchanger. In the numerical simulation, the tube flow is a hot fluid, i.e., smoke gas flow, through the CFD simulation of the fluid in the heat exchanger tube, the temperature field and velocity field of the heat exchanger tube fluid are obtained, and then these fields are analyzed and compared to optimize the original heat exchanger tube and to design a heat exchanger tube with higher efficiency.

　　3.1.1 Temperature field analysis

　　5 shown from left to right for the heat exchanger bottom, middle, top cross-section of the temperature cloud, 6 in the circular area of the heat exchanger tube central cross-section temperature distribution cloud, it can be seen that the center of the heat exchanger tube temperature is high, the wall temperature is low, and the temperature gradient is large. This is mainly due to the existence of fluid viscosity, near the wall fluid flow state for the laminar flow, and laminar flow thermal resistance, the formation of a thermal boundary layer, where the heat transfer effect needs to be further improved.7 shown in the pipe process fluid domain axial symmetry surface temperature distribution cloud, the flue gas temperature in the pipe process is gradually reduced to a certain extent, indicating that the heat transfer is more obvious.

　　3.1.2 Velocity field analysis

　　In order to further understand the heat exchanger tube flue gas flow state on the heat transfer heat exchanger, the flue gas flow velocity field is analyzed. 8, 9 shown in the heat exchanger tube transverse and longitudinal cross-section fluid velocity vectors, respectively. As can be seen from 8, the heat exchanger tube cross-section velocity distribution change is not very obvious, the velocity gradient is small; and from 9 can be seen, the fluid in the pipe course in the heat exchanger tube flow velocity first slightly increased, and then to the middle of the heat exchanger tube and the back of the flow rate is basically stabilized. In summary, it can be seen that: the flue gas fluid in the heat exchanger tube speed change is not large, the turbulence intensity is small, the heat transfer resistance is large.

　　3.2 Analysis of shell flow field

In the numerical simulation, the shell flow is cold fluid, i.e., air flow, and the shell flow structure is another focus of heat exchanger research. If the shell structure is not reasonably designed, it is easy to lead to the reduction of heat transfer efficiency, flow loss increases and other defects, so the design of reasonable shell structure, improve the heat exchanger shell flow state can effectively eliminate these defects.

　　3.2.1 Temperature field analysis

　　Shown in 10 and 11 are the temperature clouds in the longitudinal section of the shell process and at the outlet, respectively. From 10, it can be seen that the air temperature in the shell process generally increases gradually along the flow direction of the fluid, and the closer it is to the wall of the heat exchanger tube, the higher the temperature. However, careful observation, in the lower right corner of the heat exchanger and the upper left corner have a block of regional temperature than the surrounding temperature, this is because the fluid in this region is in a relatively stopped state, in which there are many small eddies, small eddies in the fluid velocity is very low, so that this area is quickly heated, and due to the stagnation, the heat can not be transferred, so the temperature of this area is relatively high, that is, the formation of the "heat transfer dead zone". From 11 it can be seen that the average temperature of the fluid at the outlet of the shell course is 91.6°C, but the temperature distribution is not uniform.

　　3.2.2 Velocity field analysis

　　12 shows the longitudinal cross-section of the shell process velocity cloud, the velocity at the above heat transfer dead zone is exactly in line with the 12 shown in the heat exchanger in the upper left corner and the lower right corner of the velocity is very low, the fluid is almost at a standstill, the air fluid from the shell process inlet into the shell process, through the heat exchanger tube of the perturbation of the flow, diagonally upward along the shell process outlet out. Therefore, appropriate measures should be taken to eliminate the "heat transfer dead zone" exists.

04

Optimization measures and results

　　4.1 Optimization measures

Take the following measures to optimize the model: ① appropriate increase in the number of heat exchanger tube, from the original 34 to 39; ② heat exchanger tube shape from the round tube into the equivalent diameter of the equivalent of the flat tube; ③ shell program to add two spacers to play the role of disturbing the flow, in order to reduce the two corners of the "heat transfer dead zone". Optimized model shown in 13.

　　4.2 Analysis of optimized results

　　The longitudinal velocity and temperature clouds of the heat exchanger after optimization are shown in 14 and 15 respectively. From 14, it can be seen that the shell process fluid from the shell process inlet, due to the expansion of the cross-section, the velocity is suddenly reduced, the fluid through the heat exchanger tube of the disturbing effect, the velocity is sometimes high and sometimes low, the formation of a more intense turbulent flow. Compared with 12, the heat transfer of the heat exchanger is promoted by the addition of two spacers in the shell course, which eliminates the stagnation zone of the fluid in the shell course.

　　From 15, it can be found that the change in fluid temperature from inlet to outlet is much more obvious than 7, indicating that the heat transfer effect of the flat tube is better than that of the round tube, and the optimization of the heat exchanger tube is reasonable; due to the role of the two spacers in the shell process, the temperature of the shell process fluid from the inlet to the outlet is a "Z" type increase, compared with 10, there is no temperature in the optimized model. Compared with 10, the optimized model has no high or low temperature area, and the average temperature of the outlet reaches 111.6℃, and the temperature distribution is uniform, which indicates that the optimization of the heat exchanger shell process structure is also reasonable.

05

Comparison between simulation results and test results

The model used for simulation is the same size as the test prototype, and the measurement position and method refer to the coal-fired hot air furnace standard JB/T6672-2011. boundary conditions used in the numerical simulation, such as the inlet flow rate of the shell and tube, inlet temperature, etc. are all from the test, and the simulation results and the test results of the comparison of the item is the outlet temperature of the shell fluid, and the magnitude of the value of which reflects the thermal efficiency of heat exchanger. The size of the value can reflect the heat exchanger thermal efficiency. Table 5 shows the comparison between the experimental and simulated values of shell outlet temperature before and after the model optimization, and both of them are in good agreement with each other, and the error is within 10%, which verifies the accuracy of the numerical simulation.

06

Conclusion

　　Through the CFD simulation of the heat exchanger heat exchanger shell process fluid and tube process fluid, respectively, the heat exchanger tube process fluid and shell process fluid flow field characteristics were carefully studied to find out the irrationality, and put forward the optimization plan, through the test to verify the accuracy of the numerical simulation and optimization of the reasonableness.