Numerical simulation of flow field and temperature separation in a vortex tube

A numerical analysis of flow field and temperature separation in a uni-flow vortex tube type is described. Effects of the turbulence modeling (k–ε model and ASM), numerical scheme (hybrid, upwind and second-order upwind) and grid density on calculation of energy separation in the vortex tube are also conducted. It is found that the calculated results are in reasonably good agreement with the experimental data for both the static and total temperatures; the use of the ASM improves slightly the accuracy of the predictions than that the k–ε model. It is also observed that larger temperature gradients appear in the outer regions close to the tube wall for the static temperatures and the separation effect or the difference of the total temperature is high in the core region near the inlet nozzle. The maximum total temperature in the field is visible at the axis location of x/D_o between 0.5 and 1.0 for the ASM.     

Heat transfer and turbulent flow friction in a circular tube fitted with conical-nozzle turbulators

The paper deals with an experimental study of the influence of conical-nozzle turbulator inserts on heat transfer and friction characteristics in a circular tube. In the present work, the turbulators are placed in the test tube section with two different types: (1) diverging nozzle arrangement (D-nozzle turbulator) and (2) converging nozzle arrangement (C-nozzle turbulator). The turbulators are thoroughly inserted inside the tube with various pitch ratios, PR = 2.0, 4.0, and 7.0. The Reynolds number based on the bulk average properties of the air is in a range of 8000 to 18,000 and the experimental data obtained are compared with those obtained from the plain tube and from the literature. The experimental results reveal that increasing the Reynolds number at a given pitch ratio of the turbulators leads to the significant increase in Nusselt number indicating enhanced heat transfer coefficient due to rising convection as the flow increases. However, the friction factor at a given Reynolds number considerably increases with the reduction of pitch ratio and Reynolds number. The D-nozzle arrangement, creating stronger reverse/turbulence flow, provides higher the heat transfer rate and friction factor than the C-nozzle arrangement. The heat transfer rates obtained from using both nozzle-turbulators, in general, are found to be higher than that from the plain tube at a range of 236 to 344%, depending on Reynolds number and the turbulator arrangements. In addition, proposed correlations from the present experimental data for Nusselt number and friction factor are also presented.     

Heat transfer behaviors in a tube with combined conical-ring and twisted-tape insert

Heat transfer, friction factor and enhancement efficiency characteristics in a circular tube fitted with conical-ring turbulators and a twisted-tape swirl generator have been investigated experimentally. The heat transfer test section is heated electrically imposing axially and circumferentially constant wall heat flux boundary conditions. In the experiments, two enhancement heat transfer devices are applied. One is the conical-ring used as a turbulator and placed in the tested tube and the other is the twisted-tape swirl generator placed at the core of the conical-ring. Air as the tested fluid is passed both enhancement devices in a Reynolds number range of 6000 to 26,000. Two twisted-tapes of different twist ratios, Y = 3.75, and 7.5, are introduced in each run. The experimental results reveal that the tube fitted with the conical-ring and twisted-tape provides Nusselt number values of around 4 to 10% and enhancement efficiency of 4 to 8% higher than that with the conical-ring alone. A maximum heat transfer rate of 367% and enhancement efficiency of around 1.96 is found for using the conical-ring and the twisted-tape of Y = 3.75. For all the devices used, the enhancement efficiency tends to decrease with the rise of Reynolds number and to be nearly uniform for Reynolds number over 16,000. In addition, correlations for Nusselt number, friction factor and performance evaluation criteria to assess the real benefits in using the turbulator and swirl generator of the enhanced tube are determined.     

Enhancement of heat transfer using CuO/water nanofluid and twisted tape with alternate axis

Heat transfer, friction and thermal performance characteristics of CuO/water nanofluid have been experimentally investigated. The nanofluid was employed in a circular tube equipped with modified twisted tape with alternate axis (TA). The concentration of nanofluid was varied from 0.3 to 0.7% by volume while the twisted ratio (y/W) of TA was kept constant at 3. The experiments were performed in laminar regime (Reynolds number spanned 830 ≤ Re ≤ 1990). The uses of nanofluid together with typical twisted tape (TT), TA alone and TT alone were also examined. To evaluate heat transfer enhancement and the increase of friction factor, the Nusselt number and friction factor of the base fluid in the plain tube were employed as reference data. The obtained results reveal that Nusselt number increases with increasing Reynolds number and nanofluid concentration. By the individual uses of TA and TT, Nusselt numbers increase up to 12.8 and 7.2 times of the plain tube, respectively. The simultaneous use of nanofluid and TA improves Nusselt number up to 13.8 times of the plain tube. Over the range investigated, the maximum thermal performance factor of 5.53 is found with the simultaneous employment of the CuO/water nanofluid at 0.7% volume and the TA at Reynolds number of 1990. In addition, the empirical correlations for heat transfer coefficient, friction factor and thermal performance factor are also developed and reported.     

Thermal performance evaluation of heat exchangers fitted with twisted-ring turbulators

Heat transfer, friction factor and thermal performance characteristics in a tube equipped with twisted-rings (TRs) are experimentally investigated. The experiments were conducted using TRs with three different width ratios (W/D = 0.05, 0.1 and 0.15) and three pitch ratios of (p/D = 1, 1.5 and 2) for Reynolds numbers ranging from 6000 to 20,000 using air as a test fluid. The typical circular rings (CRs) were also tested for an assessment. The experimental results reveal that most TRs yield lower Nusselt numbers and friction factor than CRs, except at the largest width ratio (W/D = 0.15) and the smallest pitch ratio (p/D = 1.0). In addition, Nusselt number and friction factor increase as width ratio increases and pitch ratio decreases. However, a maximum thermal performance factor is associated by TRs with the smallest width ratio and pitch ratio. The empirical correlations of the heat transfer (Nu) and friction factor (f) are also included in this paper.     

Laminar periodic flow and heat transfer in a rectangular channel with triangular wavy baffles

This paper presents a numerical analysis of laminar periodic flow and heat transfer in a rectangular constant temperature-surfaced channel with triangular wavy baffles (TWBs).The TWBs were mounted on the opposite walls of the rectangular channel with inline arrangements.The TWBs are placed on the upper and lower walls with attack angle 45?.The numerical is performed with three dif-ferent baffle height ratios (BR=b/H=0.05 0.3) at constant pitch ratio (PR) of 1.0 for the range 100 ≤ Re ≤ 1000.The computational results are shown in the topology of flow and heat transfer.It is found that the heat transfer in the channel with the TWB is more effective than that without baffle.The in-crease in the blockage ratio,BR leads to a considerable increase in the Nusselt number and friction factor.The results indicate that at low BR,a fluid flow is significantly disturbed resulting in inefficient heat transfer.As BR increases,both heat transfer rate in terms of Nusselt number and pressure drop in terms of friction factor increase.Over the range examined,the maximum Nu/Nu0 of 7.3 and f/f0 of 126 are both found with the use of the baffles with BR=0.30 at Re=1000.In addition,the flow structure and temperature field in the channel with TWBs are also reported.     

Experimental investigation of energy separation in a counter-flow Ranque–Hilsch vortex tube with multiple inlet snail entries

The energy/temperature separation phenomenon and cooling efficiency characteristics in a counter-flow Ranque–Hilsch vortex tube (RHVT) are experimentally studied. The ascertainment focuses on the effects of the multiple inlet snail entries (N = 1 to 4 nozzles), cold orifice diameter ratios (d/D = 0.3 to 0.7) and inlet pressures (P_i = 2.0 and 3.0 bar). The experiments using the conventional tangential nozzles (N = 4), are also performed for comparison. The experimental results reveal that the RHVT with the snail entry provides greater cold air temperature reduction and cooling efficiency than those offered by the RHVT with the conventional tangential inlet nozzle under the same cold mass fraction and supply inlet pressure. The increase in the nozzle number and the supply pressure leads to the rise of the swirl/vortex intensity and thus the energy separation in the tube.     

Influence of combined non-uniform wire coil and twisted tape inserts on thermal performance characteristics

In this paper, heat transfer, friction factor and thermal performance behaviors in a tube equipped with the combined devices between the twisted tape (TT) and constant/periodically varying wire coil pitch ratio are experimentally investigated. The periodically varying three coil pitch ratios were arranged into two different forms: (1) D-coil (decreasing coil pitch ratio arrangement) and (2) DI-coil (decreasing/increasing coil pitch ratio arrangement) while the twisted tapes were prepared with two different twist ratios. Each device alone is also tested and the results are subjected for comparison with those from the combined devices. The experiments were conducted in a turbulent flow regime with Reynolds numbers ranging from 4600 to 20,000 using air as the test fluid. Compared to each enhancement device, the heat transfer rate is further augmented by the compound devices. Over the range investigated, the highest thermal performance factor of around 1.25 is found by using DI-coil in common with the TT at lower Reynolds number. In addition, the empirical correlations of the heat transfer (Nu) and pressure drop (f) are also presented.     

Experimental Study of TiO2-Water Nanofluid Flow in Corrugated Tubes Mounted with Semi-Circular Wing Tapes

Heat transfer of corrugated tubes mounted with semi-circular wing tapes has been experimentally investigated. TiO₂-water nanofluids were employed as working fluids. Experiments were conducted in the following ranges: (i) TiO₂-water nanofluids with three TiO₂ concentrations (0.05 to 0.15% by volume), (ii) semi-circular wing tapes with three wing angles (45 to 75°), and (iii) two wing arrangements, and (4) Reynolds numbers between 8000 and 15,000. The experiment using water as the working fluid in a corrugated tube without tape was also performed, for comparison. Evidently, the heat transfer enhancement increases with increasing wing angle. Moreover, the corrugated tubes combined with semi-circular wing tapes in parallel arrangement show better thermal performance than the ones in counter arrangement. The highest thermal enhancement factor of 1.98 is obtained by the use of the combined devices in parallel pattern at wing angle of 75°, concentration of TiO₂-water nanofluid of 0.15% by volume and Reynolds number of 8000.     

Influence of triangular wavy baffles on heat and fluid flow characteristics in a channel

Laminar periodic flow and heat transfer in a three-dimensional channel with triangular wavy baffles (TWBs) have been numerically investigated. The baffles with three different angles of attack: 30°, 45° and 60° have been considered for Reynolds numbers ranging from 100 to 1000. For a better understanding in the heat transfer mechanisms, the pressure contour, secondary flow pattern, streamlines of impinging flow, wall streamline and iso-surface are also reported. Apparently, each wavy baffle generates two counter-rotating vortices. The 30° and 45° baffles generate vortices with comparable intensities which are considerably higher than that caused by 60° baffles. At similar conditions, the 30° and 45° baffles give comparable Nu/Nu₀ values which are considerably higher than that provided by the 60° baffles, but the 30° baffles cause lower f/f ₀ than the 45° and 60° baffles. For the range determined, the maximum thermal performances achieved by using baffles with the attack angles of 30°, 45° and 60° are 2.3, 2.2, and 1.88, respectively.