| Naan is a kind of baked food which uses flour as the main material.As the staple food of ethnic minorities in Xinjiang,China,it occupies a very important position in the daily dietary of Central Asian countries,which is widely consumed and has a broad mass base and consumption prospects.Its industrial production and transformation is imminent.As a professional baking equipment for large-scale continuous production,the tunnel oven has extremely high production efficiency and is suitable for a variety of foods,which is the best choice for industrial baking of Naan.However,complex mechanism of heat and mass transfer in Naan’s baking progress make the production difficult to control,leading to unstable baking quality.Moreover,some structure design of the tunnel oven are unreasonable,leading to poor gas flow uniformity and uneven heat transfer of food,which will reduce working efficiency and product quality.Aiming at the baking efficiency improvement of the tunnel oven and industrialization and high-quality baking of Naan,research on these key issues has carried out in this paper.The main research contents and conclusions are as follows:(1)The thermophysical property change of Naan during baking was studied.The temperature change,water content,density and dry matter’s specific heat during baking in a multifunctional electric oven were measured.According to the experimental data and theoretical derivation,the functions of the specific heat and thermal conductivity of Naan with temperature are obtained by using the water content model and inverse method respectively.(2)Based on the variation law of thermophysical properties in the baking process,the heat transfer model of Naan during baking was established.The baking process of Naan was simulated using CFD software,and the simulation results were basically consistent with the experimental results in temperature variation.The average deviations between the simulated and actual values of Naan’s surface and internal temperatures were 2.9 % and 3.1 %respectively,which validated the reliability of the heat transfer model.(3)The high working pressure of the combustion tube and strong trend of gas diffusion in the direct-combustion tunnel oven lead to poor gas flow uniformity and uneven heat transfer and unstable baking quality of food in the same batch.To solve this problem,a dual-tube and dual-segment structure is proposed in this paper,which can effectively reduce the gas pressure and alleviate the gas diffusion trend by using the inner tube structure and the intake valve to regulate the injection volume on both sides of the combustion tube.The baking process of Naan with 3 different structures of combustion tubes was simulated.The standard deviations of the average surface temperature of Naan baked with dual-tube and dual-segment structure tube in the same batch were 0.257,0.507 and 0.712,with the smallest degree of dispersion.The results showed that the smallest temperature difference and the best baking effect are in the same batch of Naan baked with dual-tube and dual-segment structure tube.(4)Structural optimization and CFD simulation were applied to hot-air circulation tunnel oven.The air intake channel with side-inlet structure of the upper and lower wind boxes was designed,with a maximum wind speed difference of 4.5 m / s at the upper and lower outlets,which effectively balanced the air volume of the two wind boxes and reduced the difference of food temperature in the vertical direction.The wind box with dense fan blade structure was proposed to balance the air volume in the width direction.The standard deviations of the average surface temperature of Naan in the same batch were 1.695,1.862 and 2.743,with a small degree of dispersion,improving the stability of food baking effect of the same batch.The air volume regulation device was installed in the air intake and return channel,which realized the free control of the air volume of the wind boxes and the fine adjustment of the overall air volume,further improving the baking quality.CFD simulation was carried out for the structure optimization,and the results showed the improvement of gas flow uniformity inside the oven.(5)The equal proportional test platform of the combustion tube of direct-combustion tunnel oven and the hot air circulation system of hot-air circulation tunnel oven were built,and the structural optimization of the tunnel oven was verified.The average deviation of wind speed on the left and right sides of the combustion tube were 16.1 %,5.8 %,1.6 % and2.3 %,respectively;the horizontal wind speed difference in the air intake channel was within± 0.3 m / s;the standard deviation of wind speed at measurement point of the wind box was0.37,0.31,and 0.29.The results show that the gas flow inside the oven is more uniform,which proves the improvement of the actual baking effect of the tunnel oven after optimization. |
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