مطالعه‌ي تجربي جابه‌جايي اجباري نانوسيالات در لوله‌ي حلزوني

Experimental Study on Force Convection of a Nanofluid in a Spiral Tube

اين پژوهش به بررسي انتقال حرارت جابه‌جايي و ضريب اصطكاك نانوسيالات مس ـ آب و آلومينيوم ـ آب در كويل حلزوني اختصاص دارد. اين آزمايش‌ها براي رژيم آرام جريان و تحت شرايط مرزي دماي ديواره ثابت انجام شده است و نانوسيالات در غلظت‌هاي مختلف و شرايط عملكرد متفاوت مورد آزمايش قرار گرفته‌اند. اثر پارامتر‌هايي همچون عدد گراتز، دماي ديواره و غلظت بر ضريب انتقال حرارت و افت فشار جريان مطالعه شده است. هدايت حرارتي به‌صورت تجربي اندازه‌گيري شده است و نتايج حاصله نشان مي‌دهد كه در غلظت ppm 2000 هدايت حرارتي نانوسيال آب ـ مس 18% بيشتر از آب ـ آلومينيوم است. به‌دليل جريان ثانويه‌ي ايجاد شده در لوله، عدد ناسلت با افزايش عدد گراتز به‌صورت نوساني تغيير مي كند. درمورد نانوسيال مس و آلومينيوم افزايش ضريب هدايت حرارتي در بهترين حالت به حدود 26 و 22 درصد مي‌رسد.

Fluid heating and cooling are important in many industries such as power, manufacturing, transportation, and electronics. Effective cooling techniques are greatly needed for cooling any sort of high-energy device. Common heat transfer fluids, such as water, ethylene glycol and engine oil, have limited heat transfer capabilities due to their low heat transfer properties, and numerous researchers have been investigating better ways to enhance the thermal performance of heat transfer fluids. Nanofluids are suspensions of nanoparticles in base fluids; a new challenge for thermal sciences provided by nanotechnology. The tested fluids are prepared by dispersing the Al and Cu or metal base fluids into the water at three different concentrations; 500, 1000 and 2000 ppm. Thermal conductivities of these fluids are measured experimentally by a thermal property analyzer, i.e., KD2 Pro, using a KS-1 sensor needle, as this needle is preferred for low viscosity fluids. Experimental results show that the thermal conductivity of nanofluids is higher than base fluid and the thermal conductivity of Cu/Water nanofluid is more than Al/Water nanofluid. This is because Cu-metal thermal conductivity is more than Al-metal thermal conductivity. In addition, a comparison is made between the experimental results of thermal conductivity and the results calculated using the models presented for their prediction. This study presents the heat transfer coefficient and friction factor of Al-water and Cu-water nanofluids flowing in a spiral coil in the laminar flow regime with constant wall temperature. The experiments were undertaken at different concentrations and under various operational conditions. The effects of different parameters, such as Gz number, wall temperature and nano?uid particle concentration, on the heat transfer coefficient and pressure drop of the flow were studied. The thermal conductivities of these fluids are measured experimentally and results show that the thermal conductivity of Cu-Water nanofluid is about 18% higher than Al-Water nanofluid in 2000 ppm. Nusselt number oscillations can be seen for different nanofluids, which are caused by the secondary flow.