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Heat transfer and thermal stress analysis in grooved tubes

Heat transfer and thermal stresses, induced by temperature differences in the internally grooved tubes of heat transfer equipment, have been analysed numerically. The analysis has been conducted for four different kinds of internally grooved tubes and three different mean inlet water velocities. Constant temperature was applied from the external surface of the tube. Energy and governing flow equations were solved using finite difference scheme. Finite element method (FEM) was used to compute the thermal stress fields. Grooving effects on the thermal stress ratio have been discussed. As a result, maximum thermal stress occurs in the case ofp =d for all water inlet velocities. The maximum thermal stress ratio positions inside the tube have been indicated as MX for all investigated cases. In the light of the thermal stress values, various designs can be applied to reduce thermal stress in grooved tubes.

Effect of thermal stratification on energy and exergy in vertical mantled heat exchanger

In the present study, effect of thermal stratification on energy and exergy storage amount in vertical mantled hot water storage tanks, which are used common in solar domestic hot water systems, has been investigated. Placing obstacle inside the tank has enhanced thermal stratification and so energy and exergy of the tank. A commercial vertical mantled hot water tank was selected for experimental study. Four different obstacles were placed in four different heights from the tank bottom. Effect of the obstacle types and positions on energy and exergy storage capacity of tank has been researched experimentally. As a result, placing obstacles inside tank has enhanced the energy and exergy of tank. The best increasing has been seen for Obstacle A in Y/H = 0.250 position. In this situation energy amount increased 1.86 times and exergy amount increased 2.02 times higher than ordinary tank.

Numerical Investigation of Thermal Performance of a Tube Fitted With Regularly Spaced Twisted Tape Elements

This work presents a numerical investigation on thermal performance of a tube fitted with regularly spaced twisted tape elements in turbulent flow regime. Two different twisted tape widths of 46 mm and 44 mm which are lower than the tube inside diameter of 50 mm are used, in order to reduce excessive pressure drops associated with full width twisted tape elements. The numerical analyses were performed with various free space ratios (S = 0.0, 1.0, and 2.0) at a constant twist ratio (TR) of 5. Uniform heat flux was applied to the external surface of the tube and Reynolds numbers varied from 5918 to 46875 in the numerical calculations. The numerical results obtained from a plain tube were compared with those from the studies in literature for validation of numerical technique. The use of twisted tape elements leads to a considerable increase in heat transfer and pressure drop over the plain tube. Consequently, the numerical results reveal that the best operating regime of all twisted tape elements was found at low Reynolds number, leading to more compact heat exchanger.© 2012 ASME

The Effect of Obstacle Geometry and Position on Thermal Stratification in Solar Hot Water Storage Tanks

In this study the effect of obstacle geometry and its position on thermal stratification in solar powered domestic hot water storage tanks are numerically investigated. The goal of this study is to obtain higher thermal stratification and supply hot water for usage as long as possible. The temperature distributions are presented for three different obstacle geometries (1, 2 and 3) and six different distances (f = 0.3, 0.4, 0.5, 0.6, 0.7 and 0.8 mm) from the bottom of the hot water storage tank. The numerical method is validated using both experimental and numerical results available in the literature. It is observed from the results that the thermal stratification increases with the increasing obstacle distance from the bottom of the hot water storage tank for obstacle 1 and 3. The obstacle 2 provides less thermal stratification than the obstacles 1 and 3. As a result, in a duration of 30 minutes, the obstacle 3 provides the best thermal stratification for the distance of f = 0.8 mm from the bottom of the hot water storage tank.Copyright

Optimum Dimensions of an Obstacle Placed in a Hot Water Tank for Thermal Stratification

Dimension of an obstacle placed in a hot water tank for thermal stratification is optimized numerically. Numerical method is validated using both experimental and numerical results. A cylindrical tank used to store heat for solar collector applications is considered. A cylindrical obstacle with a hole in the middle is placed in the tank and various f/H and g/D ratios of the obstacle geometry are considered. Here H and D are the height and diameter of the tank, respectively, where f is the distance from the bottom surface of the tank to the cold-water inlet channel and g is the diameter of the hole in the obstacle. Temperature distribution in the tank, water temperature supplied by the tank, and temperature differences at various tank inlet and outlets are obtained for various f/H and g/D ratios. The results show that placing obstacle in the tank improves thermal stratification, and thus it increases the temperature of water supplied by the tank compared with no obstacle case and that the best thermal stratification is obtained for obstacle dimensions corresponding to g/D ratio of 0.2 and f/H ratio of 0.13.