| As an important tool for winter vegetable production in northern China,greenhouses have been widely promoted and used in recent years.The optimization and transformation of greenhouses are also being carried out simultaneously.So far,the greenhouse has undergone the transformation of the third generation.On the basis of the third-generation energy-saving solar greenhouse,along with the energy and environmental protection issues that are highly valued in today's society,the ecological greenhouse of high self-sufficiency comes into being.However,the auxiliary heating problem of ecological greenhouse is still being explored.Based on the principle of smoldering heat production of agricultural biomass waste,the burning cave auxiliary heating system highly conforms to the concept of ecological greenhouse,and using it to assist the heating of ecological greenhouse,which not only makes good use of agricultural biomass waste,improves economic benefits,but also reduces environmental pollution.However,there are few research methods to control the heating rate of burning cave.In this paper,based on the above two major problems and years of greenhouse environment and burning cave heating experiment data,theoretical research,numerical simulation and experimental verification methods are adopted to study the auxiliary heating conditions and heating mode of ecological greenhouse in northern cold regions,as well as the improvement of burning cave auxiliary heating system.First,consider the outdoor temperature and the influence of solar total radiation values inside the greenhouse environment,proved the northern cold ecological hardy or half cold resistant crops in greenhouse,the ecological greenhouse need burning cave auxiliary heating of3 kinds of outdoor climatic conditions:1)when the greenhouse solar total radiation values greater than 300 W/m~2,and outside the greenhouse temperature below 20?;2)when the total solar radiation value outside the greenhouse is between 150 W/m~2 and 300 W/m~2,and the temperature outside the greenhouse is lower than-10?;3)when the total solar radiation value outside the greenhouse is less than 150 W/m~2,and the temperature outside the greenhouse is less than-5?.As well as specific 4 kinds of auxiliary heating methods of burning cave:1)the average ground temperature at 200mm below the interior of the greenhouse should be increased by 2?within 5 days;2)the average ground temperature at 200mm below the interior of the greenhouse should be increased by 4?within 5 days.3)the average ground temperature at200mm below the interior of the greenhouse by 2?within a 10-day period;4)within 20 days,the average ground temperature at 200mm below the interior of the greenhouse should be increased by 1?.Secondly,the existing auxiliary heating system of burning cave is modified and predicted by numerical simulation.The air flow in the burning cave was numerically simulated under three conditions:1)a baffle was set at the bottom of the burning cave entrance;2)no baffle in the burning cave;3)a baffle is set on the top of the burning cave.The turbulence model was Realizable k-e equation and non-equilibrium wall function.The results show that due to the different baffle positions,the air flows into the combustion cave are also different,which will lead to different types of smoldering in the burning cave,and then the smoldering rate in the burning cave can be controlled.Thirdly,computational fluid dynamics(CFD)was used to simulate the forward and opposed smoldering processes in the burning cave.In order to simplify the model,the fuel in the burning cave is treated as homogeneous heated multiphase porous medium.The heat release rates of forward and opposed smoldering were calculated by using the reverse calculation method based on CFD model.The calculation results of heat release rate show that,under the same air inlet velocity(0.1 m/s),the heat release rate of forward smoldering in the burning cave is 850W/m~3,and which of opposed smoldering is 500w/m~3.When the air inlet velocity increases to 0.3 m/s,the heat release rate of forward smoldering in the burning cave increases from 850W/m~3 to 2000 W/m~3,and the heat release rate of opposed smoldering increases from 500 W/m~3to 1000 W/m~3.It can be seen that forward smoldering produces rapid heating in the burning cave,but its temperature distribution on the roof of the burning cave is not uniform.On the contrary,opposed smolder causes the burning cave to heat up at a slower rate,but the temperature distribution on the roof of the burning cave is uniform.Finally,according to the results of numerical simulation,experimental verifications were carried out.The experimental results show that compared with the experimental results,the model can accurately predict the air flow and temperature distribution in the burning cave,and the error is about 5%?20%.The experiment results also show that the smoldering combustion in the unimproved burning cave is easy to be converted into open flame combustion,which resulting in the problem of excess heating.For the auxiliary heating of the burning cave of ecological greenhouses in northern cold regions,during most of winter,opposed smoldering should be adopted in the burning cave to assist the heating of the greenhouses.If the greenhouses encounter such adverse climates as abrupt temperature drop,snowfall and even shade,the burning cave should be switched to forward smoldering to heat the greenhouses. |
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