Veysel Ozceyhan

Experimental Investigation of Thermal Performance in a Tube With Detached Circular Ring Turbulators

This article deals with the experimental investigation of thermal performance of detached circular ring turbulators in turbulent flow regime. The experiments were conducted by four different circular rings. Two different pitch ratios (1 and 2) and two different distances (1 mm and 2 mm) at which the circular rings were detached from the tube wall were considered in the experiments. Uniform heat flux was applied to the external surface of the tube and Reynolds number was varied from 4350 to 24,920. The tube with circular ring turbulators introduced both higher heat transfer and higher pressure drop than the smooth tube, depending on pitch ratio and distance. According to the experimental results, the Nusselt number and friction factor increased with decreasing of pitch ratio and distance. Therefore, the circular ring turbulator with a pitch ratio of 1 and distance of 1 mm offered the highest heat transfer rate accompanied by the highest pressure drop. The highest thermal performance efficiency of 26% was also achieved for this case at a Reynolds number of 4421. Evidently, the experimental results revealed that the best operating regime of all circular ring turbulators was found at low Reynolds number, leading to a more compact heat exchanger.

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.

Heat Transfer Enhancement in a Tube Using Triangular Ribs

A numerical study was undertaken for investigating the heat transfer enhancement in a tube with triangular cross sectioned ribs. The spacing between the ribs were kept constant as a distance of tube diameter, D. Three different rib thicknesses were considered for numerical analyses. Uniform heat flux was applied to the external surface of the tube and air was selected as working fluid. Numerical calculations were performed with FLUENT 6.1.22 code, in the range of Reynolds number 8000–36000. The results obtained from a smooth tube and rib inserted tube were compared with those from the experimental studies in literature in order to validate the numerical method. The variation of Nusselt number, friction factor and overall enhancement ratios for the tube with triangular cross sectioned ribs were presented. Consequently, a maximum gain of 1.34 on overall enhancement ratio is obtained for S/D = 0.75.Copyright

Particle-Tracking Random-Walk Computation of High-Peclet-Number Convection

A numerical method based on particle tracking and random walk is applied to convective heat transfer. The technique is useful in problems with high Peclet numbers in which the velocity field is known from other means. Advection is simulated by particle motion along a pathline, and diffusion by random walk. The advection time and diffusion distance determine the Peclet number. Several examples of the computation of temperature field and wall heat fluxes in advection-dominated flows for which analytical solutions are known are provided as illustrations of the method.

NUMERICAL ANALYSIS OF HEAT TRANSFER ENHANCEMENT IN CONICAL-NOZZLE TURBULATORS INSERTED TUBE

This numerical work is associated with the investigation of the effect of turbulator arrangement (diverging and converging) on heat transfer enhancement in conical nozzle turbulators inserted circular tube. Three different pitch ratios (PR=2.5, 5 and 7.5) were considered in the numerical analyses, using air as test fluid. The numerical results of Nusselt number, friction factor and thermal performance factor were introduced for the range of Reynolds numbers of 6000-24000. The obtained results showed that smaller pitch ratios provided more heat transfer rate and pressure drop. As compared to converging nozzles, diverging nozzles generated more effective turbulance/reverse flow, and thus provided higher heat transfer and flow friction. Evidently, obtained results revealed that the usage of nozzle-turbulators as a turbulence/reverse flow generator has an important contribution on heat transfer improvement.

Optimization of heat transfer and pressure drop in a tube with loose-fit perforated twisted tapes by Taguchi method and grey relational analysis

This work introduces the determination of the optimum values of the design parameters in a tube with loose-fit perforated twisted tapes. The effects of the design parameters such as twist ratio (y/D), width ratio (W/D), hole diameter ratio (d/D) and Reynolds number (Re) on heat transfer (i.e. Nusselt number) and pressure drop (i.e. friction factor) were analyzed by Taguchi method (TM) and grey relational analysis (GRA). The Nusselt number and friction factor were taken into account as performance parameters. Taguchi Method is based on analysis of variances and implements the orthogonal arrays for experimental design. L16 orthogonal array was selected as experimental plan. Firstly, each performance parameter was optimized, independently. Then, all the performance parameters were optimized together by TM and GRA. According to the experimental plan results, the most important factor for both Nusselt number and friction factor is Reynolds number, while the least significant factors are twist ratio (y/D) and width ratio (W/D).

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.