یکی از عوامل موثر در رشد گیاهان، وجود دمای مناسب و یکنواخت در گلخانه ها می باشد لذا به منظور دستیابی به این مهم و صرفه جویی در مصرف انرژی، استفاده از نانوسیالات و بهینهسازی سیستمهای گرمایشی می تواند جایگزین مناسبی برای سیستمهای گرمایشی پرمصرف و غیر یکنواخت باشد. برای این منظور در این تحقیق از پیکربندی گرمایش از کف و کاربرد نانوسیال برای گلخانه استفاده شده است. ابتدا هندسهی حجم کنترل یک گلخانه در نرمافزار سالیدورک طراحی و به منظور بررسی شبیهسازی، هندسهی هرمیشکل برای آن در نظر گرفته شد. شرایط مرزی برای سردترین دمای شبانه روز در سال با توجه به شرایط محیطی با اندازهگیری داده های دما، رطوبت و جریان هوا استخراج گردید. تحلیل پارامتریک نتایج برای دو نوع نانوسیال آلومینا و دیاکسید تیتانیوم در درصدهای حجمی 1، 2 و 3 درصد صورت پذیرفت و تاثیر پارامترهای مذکور بر عدد ناسلت و در جریان گرمایش از کف مورد بررسی قرار گرفت. نتایج نشان داد که نانوسیال آلومینا نسبت به نانوسیال دیاکسید تیتانیوم دارای عدد ناسلت بیشتری بوده و افزایش قطر نانوذرات تاثیر منفی بر مقدار عدد ناسلت دارد. همچنین با مقایسهی ضریب عملکرد گرمایش از کف گلخانهی هرمی شکل بین نانوسیال آلومینا و نانوسیال دیاکسید تیتانیوم مشاهده میشود که متوسط مقدار این ضریب برای نانوسیال آلومینا 14 درصد بیشتر است.
عنوان مقاله [English]
Simulation of greenhouse floor heating and investigating the effect of nanofluids in it
In recent years, Underground heating systems are one of the cleanest and best types of heating systems which these techniques have been used in many greenhouses. In this method, a source of thermal energy, which is often a gas or diesel, is used to heat the fluid. Then, the heated fluid is transferred to the entire greenhouse through the pipe networks that are placed on the floor of the greenhouses and under the soil, and creates a pleasant heat. During the cold months of the year, having a proper heating system for the greenhouse is essential. A standard greenhouse heating system could improve the temperature inside the greenhouse and spread it evenly on the entire surface of the greenhouse and finally, it is very effective in the growth and quality of plants and products in all months of the year. Today, fluids play a very important role in industry, especially in heating systems. Common fluids such as water, ethylene glycol and motor oil have a limited conductivity coefficient. Therefore, using the above-mentioned fluids at high temperatures causes heat transfer problems. Nanofluids consist of very small particles (usually less than 400 nm) dispersed in a base fluid. The conducted research shows that due to the high thermal conductivity of nanofluids compared to common fluids, in the future nanofluids will become a new type of fluid used in advanced heat transfer for engineering applications. Therefore, according to the importance of this topic, in this research, the heating system of the greenhouse floor is simulated and analyzed using CFD technique.
Materials and Methods In this research, in order to simplify the process of simulation, the inhomogeneity in the fluid flow is ignored and the single-phase flow is considered. In order to investigate the effect of each of the nanofluids on the fluid behavior and heat transfer of the pyramidal greenhouse, analysis and simulation of the greenhouse was performed based on three-dimensional computational fluid dynamics. First, the geometry of the control volume of a greenhouse was designed in Solidwork software, and in order to check the simulation, a pyramidal geometry was considered. The boundary conditions for the coldest day and night temperature in the year were extracted according to the environmental conditions by measuring the data of temperature, humidity and air flow. Two parameters of pressure drop value and Nusselt number were selected as target parameters in this research. The flow friction coefficient in the floor heating section was calculated through the pressure drop along the section and its hydraulic diameter. Single-phase fluid pressure drop in all pipes inside the thermal cycle was modeled in this section. Finally, the parametric analysis of the results and the comparison of the heating efficiency of the greenhouse floor for two types of nanofluid alumina and titanium dioxide in volume percentages of 1%, 2% and 3% were used. Besides, the effect of the mentioned parameters on the Nusselt number and in the flow of floor heating was investigated.
Results and Discussion
Based on the obtained results, it was concluded that an increase in Reynolds number in all volume percentages leads to an increase in Nusselt number and alumina nanofluid has a higher Nusselt number than titanium dioxide nanofluid. Also, in both nanofluids assuming a constant inlet temperature of 40℃ and a diameter of nanoparticles of 5 nm, the Nusselt number also increased with an increase in the volume percentage of particles at a constant Reynolds number. According to the results obtained with the increase in the diameter of nanoparticles, the Nusselt number decreased for both alumina and titanium dioxide nanofluids, which is greater for titanium dioxide nanofluids. Considering the findings related to the pressure drop, with the increase in the volume percentage of nanoparticles in both nanofluids, the pressure drop increased, and this drop is more severe in the alumina nanofluid, and it could be attributed to the higher density and viscosity of the alumina nanofluid compared to the titanium dioxide nanofluid. The results related to the pressure drop showed that, with the increase in the volume percentage of nanoparticles in both nanofluids, the pressure drop increased, and this drop is more intense in the alumina nanofluid and this factor is attributed to the higher density and viscosity of alumina nanofluid compared to titanium dioxide nanofluid. On the other hand, the increase in Reynolds number in both nanofluids has resulted in an increase in pressure drop. The results related to the changes in the friction coefficient in terms of Reynolds number in different volume percentages show that the coefficient decreases with the increase in Reynolds number, and these changes are more intense at lower Reynolds numbers. By comparing the performance coefficient between alumina nanofluid and titanium dioxide nanofluid, it can be concluded that the average value of this coefficient is 14% higher than other nanofluids for alumina nanofluid. But, the sensitivity of the performance coefficient of titanium dioxide nanofluid compared to alumina nanofluid is more intense to the changes of Reynolds number.
Conclusion Due to the production of greenhouse products in all seasons and the necessity of precise greenhouse control, it can be concluded that dealing with new and advanced methods in the management and optimization of the country's greenhouses is importance. The results of the present research show the fact that the simulation of heating from the greenhouse floor and its various aspects can be a suitable measure to check the uniformity and proper distribution of heat inside the greenhouse. In order to improve the efficiency of thermal equipment, using nanofluids with higher thermal ability is essential. Besides, comparing the performance coefficient of the system due to the use of nanofluids indicated the high efficiency of the use of nanofluids in comparison with pure water in the greenhouse floor heating system.