Laminar Convection Heat Transfer Enhancement Research Articles

The mechanism of laminar convective heat transfer enhancement enforced by twisting of elliptical tube

Nusselt number has unique characteristics under uniform wall temperature (UWT) and uniform wall heat flux (UWHF) thermal boundary conditions in twisted elliptical tube. Twisting of elliptical tube produces larger local Nusselt number under UWT than that under UWHF in some regions. In order to discover the mechanisms to form such characteristics in twisted elliptical tube, the velocity contribution and velocity gradient contribution to the transport of the heat flux defined by Fourier's law, q, are numerically investigated in this paper. Velocity gradient under UWHF contributes more greatly to the transport of the heat flux under UWHF than under UWT, which results larger local Nusselt number under UWHF than under UWT in straight elliptical tube. Velocity and its gradient contributions to the transport of the heat flux nearly normal to the wall surface have larger values under UWHF than under UWT in most regions, which produces larger local Nusselt number in twisted elliptical tube under UWHF than under UWT in these regions. In some regions velocity and its gradient contributions to the transport of the heat flux nearly normal to the wall surface have larger values under UWT than under UWHF produces larger local Nusselt number in twisted elliptical tube under UWT than under UWHF in these regions.

Enhancing convective heat transfer in laminar and turbulent flow regions using multi-channel twisted tape inserts

Numerical analysis of the thermal and fluid behaviors of the tubes with multi-channel twisted tapes has been investigated under constant wall temperature condition. The multi-channel twisted tapes were arranged periodically in the tubes. Twist ratio (y/w) of tape was varied from 2.0 to 4.0 and channel number (N) of tape number of channel was varied from 2 to 8. The study was carried out in both laminar and turbulent flow regions (800 ≤ Re ≤ 2000 and 5000 ≤ Re ≤ 15,000) using water as the working fluid. The finite volume method with the laminar model was applied for a laminar flow while the RNG k-ε turbulence model was applied for a turbulent flow. The numerical results show that the multi-channel twisted tapes induce multi-swirling flows which help to improve fluid mixing and enhance heat transfer rate in the tubes with the tapes as compared to those in a plain tube. For the laminar flow, the maximum thermal performance factor of 7.28 is obtained by using the tube with the multi-channel twisted-tape with N = 2 and y/w = 2.5 at Reynolds number of 2000. For turbulent flow, the maximum thermal performance factor of 1.04 is obtained by using the tube with multi-channel twisted tapes with N = 2 and y/w = 3.0 at the lowest Reynolds number of 5000. These results provide evidence that the use of multi-channel twisted tape is a promising technique for laminar convection heat transfer enhancement. It is possible to gain promising tradeoff between enhanced heat transfer and increased friction by selecting the multi-channel twisted tape with proper geometries.

Numerical studies on thermo-hydraulic characteristics of laminar flow in a heat exchanger tube fitted with vortex rods

An analysis of the thermo-hydraulic characteristics of a heat exchanger tube fitted with a novel vortex rod insert has been conducted numerically under uniform heat flux conditions. The vortex rods had three relative rod length ratios (LR = b/D = 0.15, 0.25, and 0.35) and three relative rod pitch ratios (PR = p/D = 1, 1.5, and 2), and they were studied for numerous Reynolds numbers (Re) that ranged from 300 to 1800 using water as the working fluid. With these vortex rods, longitudinal swirling (or vortex) flow can be generated for better fluid mixing between the wall and core flow regions, thereby allowing for a high heat transfer rate with a moderate increase in the pressure drop. The results indicate that the presence of the vortex rods elicits a considerable heat transfer enhancement in the tube, Nu/Nu0 = 1.1–3.9, with an increase in pressure drop, and f/f0 = 1.4–5.3. Both the Nu/Nu0 and the f/f0 ratios increase with Re and LR increases and with PR decreases. The performance evaluation criterion (PEC) values of all vortex rod cases are above unity except for the cases with LR = 0.15 at Re = 300. The case at which LR = 0.35 and PR = 1 provides the maximum PEC value of approximately 2.22 at Re = 1800. These results provide evidence that the vortex rod insert is a promising technique for laminar convection heat transfer enhancement.

Experimental Study of Laminar Convective Heat Transfer and Pressure Drop of Cuprous Oxide/Water Nanofluid Inside a Circular Tube

Laminar convective heat transfer enhancement of cuprous oxide (Cu2O)/water nanofluid flowing through a circular tube was investigated experimentally in the present work. A continuous closed loop was designed to measure heat transfer coefficients and pressure drop associated with the flow of Cu2O/water nanofluid over a wide range of laminar flow conditions. Comparison of the nanofluid experimental results with those of pure water have shown significant enhancement for heat transfer coefficients. On average, a 10% increase in heat transfer coefficient was observed with 16% penalty in pressure drop.

Numerical study on laminar convective heat transfer enhancement of microencapsulated phase change material slurry using liquid metal with low melting point as carrying fluid

The laminar convective heat transfer enhancement of microencapsulated phase change material slurry using liquid metal with low melting point as carrying fluid (MEPCM-LM slurry) in a tube with constant heat flux has been investigated numerically. The influence of each factor affecting the heat transfer enhancement was analyzed in detail by using an effective specific heat capacity model and considering axial heat conduction to be not neglected. Furthermore, the heat transfer ability of MEPCM-LM slurry was compared with that of MEPCM slurry using water as carrying fluid (MEPCM-Water slurry). It is found that bulk Stefan number and MEPCM concentration are the most important parameters influencing the heat transfer enhancement of MEPCM-LM slurry. The degrees of heat transfer enhancement increase with decreasing bulk Stefan number, dimensionless initial supercooling and/or dimensionless phase change temperature range, and increase with increasing mass concentration. The peak of the degree of heat transfer enhancement decreases with increasing Re. Moreover, the heat transfer ability of MEPCM-LM slurry is better than that of MEPCM-Water slurry, and the difference of the heat transfer ability for two slurries increases with decreasing x/r0, therefore, it is more advantageous to heat dispersion of chip in narrow space for MEPCM-LM slurry.

A numerical study on heat transfer and friction factor characteristics of laminar flow in a circular tube fitted with center-cleared twisted tape

Twisted tape is a widely used technique for heat transfer enhancement. In the present paper, we proposed a center-cleared twisted tape aiming at acxhieving good thermohydraulic performance. A comparative study between this type and the short-width twisted tape was performed numerically in laminar tubular flows. The computation results demonstrated that the flow resistance can be reduced by both methods; however, the thermal behaviors are very different from each other. For tubes with short-width twisted tapes, the heat transfer and thermohydraulic performance are weakened by cutting off the tape edge. Contrarily, for tubes with center-cleared twisted tapes, the heat transfer can be even enhanced in the cases with a suitable central clearance ratio. The thermal performance factor of the tube with center-cleared twisted tape can be enhanced by 7–20% as compared with the tube with conventional twisted tape. All these demonstrated that the center-cleared twisted tape is a promising technique for laminar convective heat transfer enhancement.