Thermal applicability of two-phase thermosyphons in ... - LEPTEN

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718 A.K. da Silva, M.B.H. Mantelli / Applied Thermal Engineering 24 (2004) 717–733NomenclatureA area, m 2b thickness, mC global conductance, W m 1 K 1c p specific heat, J kg 1 K 1F view factorg gravity, m s 2G yield radiation coefficienth heat transfer coefficient, W m 2 K 1H height, mI current, Ak thermal conductivity, W m 1 K 1L length, mNu L average Nusselt numberP experimental power applied, Wq 0 heat flux, W m 1^q dimensionless heat fluxQ heat transfer rate, WPr Prandtl numberRe Reynolds numbert time, s^t dimensionless timeT temperature, °CT average temperature, °CU overall heat transfer coefficient, W m 2 K 1v characteristic velocity, m s 1V volume, m 3 , voltage, Vx, y, z Cartesian coordinates, mGreek symbolsa absorptivity, thermal difussivity, m 2 s 1e emissivitym kinematic viscosity, m 2 s 1q reflectivityr Stefan–Boltzmann constant, W m 2 K 4Subscripts1, 2, 3, 4 internal surfaces indexair enclosed airavg, e average externalavg, i average internale evaporator
A.K. da Silva, M.B.H. Mantelli / Applied Thermal Engineering 24 (2004) 717–733 719c condenserconv convectivefilm film temperaturei; j; ...; N surface general indexI insulationL characteristic lengthms metal sheetout outside the cooking chamberrad radiative1. IntroductionEnergy generation and conservation has received special attention all over the world. This is themain concern of several industries that are forced to save energy and reduce the production costs[1]. Good examples are the companies that produce electric bakery ovens, since the energy consumptionin such devices is an important issue, especially for small business. Besides this economicfactor, which can be related to performance, functionality is also a major issue. The thermalcharacteristics of the actual ovens do not provide a uniform temperature distribution inside thecooking chamber. Usually these ovens have an electrical resistance located inside the cookingchamber. The resistance (i.e., heat source) has a small heat transfer area when compared with thevolume to be heated, which creates over-heated regions. Spreading out the heat sources inside theovenÕs cooking chamber can easily solve this problem, although it is too expensive.As an alternative solution, big blowers are installed inside the cooking chamber in order to turnthe temperature uniform. However, big electric motors consume lots of energy itself, and in spiteof that it is known that radiation is the major heat transfer mechanism inside enclosures [2–9].Carvalho and Martins [10] performed a very specific numerical study considering a threedimensionalturbulent flow of air and steam inside a cooking chamber, emphasizing the heat andmass transfer modes. They stressed the importance of the radiative heat flux as a major mechanism,showing that the radiation represents more than 70% of the total energy absorbed by thebread dough inside the oven at a cooking temperature equal to 200 °C. Based on that we canconclude that the ideal situation is the one in which the heat sources inside the cooking chamberhave large areas and temperatures slightly greater than the ideal cooking temperature. Based onsuch conclusion, how can we come up with a solution thermally and economically viable?To answer this question, the Satellite Thermal Control Group (NCTS) is studying the use of twophasethermosyphons in ovens working as a heat transfer device, which connects thermally thecombustion and cooking chambers. Several studies have been reported demonstrating the successfulapplicability of thermosyphons in other applications [11–16]. However, for the first time, verticalthermosyphons with an unusual geometry (i.e., thermosyphons where the condenser is five timeslarger than the evaporator in length) have been used in ovens. A preliminary study by Mantelli et al.[17] showed that the new geometric configuration used in the present study, which is required to athermosyphon to ‘‘fit’’ inside a convectional oven, presents a good thermal performance.
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