Highest Thermal Performance Factor Research Articles

Experimental study of heat transfer enhancement in a novel cylindrical heat sink with helical minichannels

This paper presents investigation of heat transfer and fluid flow characteristics of water in a novel cylindrical heat sink with helical minichannels experimentally. The heat sinks comprised of 36 minichannels with helix angles of 45, 60 and 90 (straight minichannels) deg, in addition the experimental tests have been carried out in the laminar flow regime (113 < Re < 478). The experimental results indicate that the decrease of helix angle and increase of Reynolds number, enhance the heat transfer of heat sink. However, the pressure drop increases with decreasing of helix angle and the increase of Reynolds number. The maximum enhancement of Nusselt number in helix angle 45 and 60 are 77.4% and 39.5% greater compared to that straight minichannel, respectively; moreover, the overall thermal resistance in highest Reynolds number decreases 50.9% and 32.2% for helix angle of 45 and 60 deg respectively. The highest thermal performance factor is obtained for the helix angle of 45 deg and highest Reynolds number. Finally, two correlations are proposed for friction factor and Nusselt number.

Heat Transfer Augmentation in Round Tube with Diamond-shaped Baffle Inserts

Influences of diamond-shaped baffle (DSB) insertion in a heat exchanger tube on its thermal performance are experimentally presented. The DSB elements were repeatedly mounted into the tube using two straight small rods to connect the DSBs together to produce the longitudinal vortex flows inside. The experiments were carried out in a constant surface heat-fluxed tube equiped with DSBs. The DSBs were placed periodically with an attack angle of 45° and a pitch spacing of three times of tube diameter (PR=P/D=3) while four different DSB heights called blockage ratios (BR=b/D=0.1, 0.2, 0.3 and 0.4) were offered. Air was used as the test fluid flowed into the tube to yield the Reynolds number (Re) in a range of 4190-26,000. The heat transfer rate and the pressure drop of the current work were presented in terms of Nusselt number (Nu) and friction factor (f), respectively. The experimental results showed that the DSB provides the increase in the heat transfer around 3.53-4.16 times higher than the smooth tube while friction factor increases around 16.68-29.52 times. To evaluate the gain of using the DSB, the thermal performance factor (TEF) is determined in a range of 1.25-1.55 whereas the highest TEF is found at BR=0.1.

Numerical investigation on heat transfer performance and flow characteristics in a roughened vortex chamber

In this study, an investigation of a vortex chamber was carried out to gain a full understanding of the nature of the vortex flow and the cooling capability inside the chamber. The paper discusses the effects on flow and heat transfer rates when the inside surface of the vortex chamber was roughened by adding flow turbulators to its wall. The turbulators took the shape of a rib with a square cross-section, the dimension of which varied between 0.25 mm and 2.00 mm. The paper also presents the results of a comparative investigation of jet impingement and vortex cooling on a concave wall using different parameters, such as the total pressure loss coefficient, Nusselt number and thermal performance factor, to evaluate the cooling effectiveness and flow dynamics. Furthermore, the entropy generation in swirl flow with the roughened wall was assessed over a wide range of Reynolds numbers.The results show that surface roughness considerably influences the velocity distribution, heat transfer patterns and pressure drop in the vortex chamber. The highest thermal performance factor takes place at rib heights of 0.25 mm and 0.50 mm with a low Re number. Further increase in rib height has an adverse impact on thermal performance. At a Reynolds number lower than 50,000, it is highly recommended to use roughened vortex cooling to obtain the best thermal performance.

Numerical Study of Heat Transfer Enhancement of Internal Flow Using Double-Sided Delta-Winglet Tape Insert

A numerical study was performed to investigate the thermal performance characteristics of an enhanced tube heat exchanger fitted with punched delta-winglet vortex generators. Computational fluid dynamics modeling was applied using the k–ε renormalized group turbulence model. Benchmarking was performed using the results of the experimental study for a similar geometry. Attack angles of 30°, 50°, and 70° were used to investigate the heat transfer and pressure drop characteristics of the enhanced tube. Flow conditions were considered in the turbulent region in the Reynolds number range of 9100 to 17,400. A smooth tube was employed for evaluating the increment in the Nusselt number and the friction factor characteristics of the enhanced tube. The results show that the Nusselt number, friction factor, and thermal performance factor have a similar tendency. The presence of this insert offers a higher thermal performance factor as compared to that obtained with a plain tube. Vortex development in the flow structure aids in generating a vortex flow, which increases convective heat transfer. In addition, as the angle is varied, it is observed that the largest attack angle provides the highest thermal performance factor. The greatest increase in the Nusselt number and friction factor, respectively, was found to be approximately 3.7 and 10 times greater than those of a smooth tube. Through numerical simulations with the present simulation condition, it is revealed that the thermal performance factor approaches the value of 1.1. Moreover, the numerical and experimental values agree well although they tend to be different at high Reynolds number conditions. The numerical and experimental values both show similar trends in the Nusselt number, friction factor, and thermal performance factor.

Double-sided delta-wing tape inserts to enhance convective heat transfer and fluid flow characteristics of a double-pipe heat exchanger

One of the challenges in the development of heat exchangers is to minimize their energy consumption. This can be achieved by enhancing the heat transfer rate, which will boost the thermal performance of heat exchangers. Delta-wing vortex generators fitted in a double-pipe heat exchanger will generate vortices with more intermixing of the flowing fluid, and provide the improvement of convective heat transfer of the internal flow. In this study, double-sided delta-wing (T-W) tape inserts were designed to enhance convective heat transfer of a double-pipe heat exchanger. The effects of the wing-width ratio (0.31, 0.47, and 0.63) on the heat transfer and fluid flow characteristics of the heat exchanger were investigated by experiments where water was used as the working fluid and the Reynolds number was varied from 5300 to 14,500. The results were compared with those obtained for a plain tube and tube with longitudinal strip (L-S) insert. The T-W tape insert (wing-width ratio: 0.63) results in the highest average Nusselt number, where the average Nusselt number is higher by 177% relative to that for the plain tube. Despite the significant heat transfer enhancement, the friction factor is 11.6 times higher relative to that for the plain tube, indicating that friction loss is more pronounced due to the presence of T-Ws. The T-W tape insert (wing-width ratio: 0.63) also results in the highest thermal performance factor (1.15). The Nusselt number, thermal performance factor, and friction factor of the heat exchanger increases as the wing-width ratio of the T-Ws increases. Based on the experimental data, empirical correlations were developed to predict the Nusselt number and friction factor of the double-pipe heat exchanger with T-W tape inserts.

Flow and heat transfer characteristics in rectangular channels using combination of convex-dimples with grooves

In this paper, the main objective is to reveal the usefulness of placing convex dimples upstream of grooves on the improvement of the thermal performance and to address the effects of convex dimple spanwise arrangements and its number on the heat transfer and friction characteristics in the convex-dimple-grooved channel. Additionally, a study on the transverse-protrusion-grooved channel is conducted to serve as a contrast, and three spanwise positions of the convex dimple as well as two convex dimple numbers are considered. The results indicate that compared with transverse protrusions, placing convex dimples upstream of grooves can produce counter-rotating vortex pairs which cause an upstream movement of the flow reattachment. In addition, vortexes shedding from convex dimples can transport the cooler central core flows to the near-wall region and wash up the hotter flows trapped inside the groove region in upward and spanwise directions, which causes significantly higher streamwise, spanwise and downward velocities. Replacing transverse protrusions with convex dimples can enhance heat transfer rate, except the case with convex dimples which are too close to side walls. All cases with convex dimples provide the significantly lower friction loss but higher thermal performance factor than that of the case with transverse protrusions. Additionally, for the convex dimple number studied here, increasing the number of convex dimples leads to a significant increase in Nusselt number as well as thermal performance factor but a slight increase in friction factor. Besides that, the spanwise shift of convex dimples shows complex results.

Heat transfer and fluid flow characteristics of heat exchanger tube with multiple twisted tapes and solid rings inserts

Heat transfer and fluid flow characteristic of a roughened tube is investigated for constant heat flux. The inserts were employed for the swirl flow generation in the test section through the tube. Pitch ratios (l/D)=1 and 2 of the solid ring tubular (SRT) and twist ratios (y/W)=2, 3 and 4 of the twisted tapes (TT) are introduced into the core flow. The Reynolds number covered in the range of 6300–22500 for co-twist arrangements for the turbulent flow of air in the tested heat exchanger tube. The study reveals that the Nusselt number, friction factor and thermal performance factor of the integrated SRT and TT are in the range of 107–293, 0.93–0.99 and 1.46–1.61, respectively. The highest thermal performance factor in the order of 1.61 over smooth tube heat exchanger was obtained for the integrated device consisting of the SRT with l/D=1.0 and y/W=2. The quadruple co-twisted tape insert provides the maximum Nusselt number, friction factor, thermal performance factor. The correlations for the Nusselt number, friction factor and thermal performance factor of the tubes with combined devices were also developed in terms of flow and geometrical parameters.

A numerical comparison of thermal performance of in-line pin–fins in a wedge duct with three kinds of coolant

This paper presents a numerical comparison of the flow and heat transfer characteristics of in-line pin–fins in a wedge duct with three kinds of coolant, including air, steam and mist/steam. The Reynolds-averaged Navier–Stokes equations, coupled with a fully-developed Shear Stress Transport (SST) turbulent model, are adopted and solved. Reynolds numbers (Re) of 10,000, 20,000, 30,000, 40,000 and 50,000 are examined to determine the effect of flow parameter on the pressure drop and heat transfer. The variation in the endwall and pin–fins area-averaged Nusselt numbers with the variation in the Reynolds numbers is obtained and compared with the experimental data. Results of the comparison indicate that the case of air coolant with a 26K temperature difference agrees satisfactorily with the experiments and shows a better heat transfer than that of a 50K temperature difference. The endwall area-averaged Nusselt numbers of steam coolant increase with the Reynolds numbers. The case of mist/steam as coolant indicates that the endwall area-averaged Nusselt numbers first increase and then decrease with Reynolds numbers and reaches its maximum at Re=40,000. Conclusively, compared with air and steam coolants, mist/steam coolant produces a lower friction coefficient and a higher thermal performance factor, which significantly improve the heat transfer enhancement of the disturbed pin–fins arrayed in the wedge duct.

Heat transfer enhancement by helically twisted tapes inducing co- and counter-swirl flows

An experimental investigation has been conducted for determining heat transfer enhancement by inserting helically twisted tapes, to induce co- and counter-swirl flows, (the tapes are symbolized as co-HTT and C-HTT, respectively). Tape pitch ratio (p/D) was varied between 1.0 and 2.0, while tape width ratio (w/D) and twist ratio (y/w) were fixed at 0.2 and 3.0, respectively. The experiments were performed for fully developed turbulent flow with Reynolds number range (Re) between 6000 and 20,000, under uniform wall heat flux condition. At similar conditions, the use of Co-HTT results in lower Nusselt number and friction factor but higher thermal performance factor than that of C-HTT. Nusselt number and friction factor increase with decreasing pitch ratio, while thermal performance factor possesses opposite trend. In addition, the empirical correlations for Nusselt number, friction factor and thermal performance factor as functions of the Reynolds number (Re), Prandtl number (Pr) and tape pitch (p/D), were developed through a multi-variant linear normal regression.

Experimental investigation of diameter effect on heat transfer performance and pressure drop of TiO2–water nanofluid

In this paper, an experimental study performed to investigate the convection heat transfer characteristics in fully developed turbulent flow of TiO2–water nanofluid. The effect of mean diameter of nanoparticles on the convective heat transfer and pressure drop studied at nanoparticle volume concentration from 0.01 to 0.02 by volume. The experimental apparatus is a horizontal double tube counter-flow heat exchanger. The nanoparticles of TiO2 with diameters of 10, 20, 30 and 50nm dispersed in distilled water as base fluid. The results indicated higher Nusselt number for all nanofluids compared to the base fluid. It is seen that the Nusselt number does not increase by decreasing the diameter of nanoparticles generally. In this study both Nusselt number and pressure drop were considered in definition of thermal performance factor. The results show that nanofluid with 20nm particle size diameter has the highest thermal performance factor in the range of Reynolds number and volume concentrations were studied.

An Experimental Investigation of Structured Roughness Effect on Heat Transfer During Single-Phase Liquid Flow at Microscale

The effect of structured roughness on the heat transfer of water flowing through minichannels was experimentally investigated in this study. The test channels were formed by two 12.7 mm wide × 94.6 mm long stainless steel strips. Eight structured roughness elements were generated using a wire electrical discharge machining (EDM) process as lateral grooves of sinusoidal profile on the channel walls. The height of the roughness structures ranged from 18 μm to 96 μm, and the pitch was varied from 250 μm to 400 μm. The hydraulic diameter of the rectangular flow channels ranged from 0.71 mm to 1.87 mm, while the constricted hydraulic diameter (obtained by using the narrowest flow gap) ranged from 0.68 mm to 1.76 mm. After accounting for heat losses from the edges and end sections, the heat transfer coefficient for smooth channels was found to be in good agreement with the conventional correlations in the laminar entry region as well as in the laminar fully developed region. All roughness elements were found to enhance the heat transfer. In the ranges of parameters tested, the roughness element pitch was found to have almost no effect, while the heat transfer coefficient was significantly enhanced by increasing the roughness element height. An earlier transition from laminar to turbulent flow was observed with increasing relative roughness (ratio of roughness height to hydraulic diameter). For the roughness element designated as B-1 with a pitch of 250 μm, roughness height of 96 μm and a constricted hydraulic diameter of 690 μm, a maximum heat transfer enhancement of 377% was obtained, while the corresponding friction factor increase was 371% in the laminar fully developed region. Comparing different enhancement techniques reported in the literature, the highest roughness element tested in the present work resulted in the highest thermal performance factor, defined as the ratio of heat transfer enhancement factor (over smooth channels) and the corresponding friction enhancement factor to the power 1/3.

Heat transfer augmentation by helically twisted tapes as swirl and turbulence promoters

This paper describes heat transfer enhancement attributed to helically twisted tapes (HTTs). Each helically twisted tape was fabricated by twisting a straight tape to form a typical twisted tape then bending the twisted tape into a helical shape. The experiments were performed using HTTs with three twist ratios (y/W) of 2, 2.5 and 3, three helical pitch ratios (p/D) of 1, 1.5 and 2 for Reynolds number between 6000 and 20,000. The conventional helical tape (CHT) was also tested for comparison. The obtained results reveal that at similar conditions (y/W and p/D), HTTs give lower Nusselt number and friction but higher thermal performance factor than CHTs. Heat transfer rate and friction factor increase as the tape twist ratio and helical pitch ratio decrease, while the thermal performance shows opposite trend. In the present range, the highest thermal performance factor of 1.29 is achieved by utilizing the tape with the largest twist ratio (y/W=3) and helical pitch ratio (p/D=2) at Reynolds number of 6000.

Thermal Performance Assessment of Turbulent Tube Flow Through Wire Coil Turbulators

This paper presents characteristics of turbulent convective heat transfer in a tube fitted with wire coil turbulators. Two different wire coils are introduced: (1) with typical/uniform coil pitch ratio (CR) and (2) with periodically varying three-coil pitch ratio. Various uniform coil pitch ratios (CR = 4, 6, and 8) and two periodically varying coil pitch ratios, the D-coil (decreasing three-coil pitch ratio arrangement) and DI-coil (decreasing/increasing three-coil pitch ratio arrangement), are experimentally investigated in a uniform heat flux tube. The experiments are performed for turbulent flows with Reynolds numbers ranging between 4500 and 20,000. All of the experimental results are compared with those obtained from using the plain tube, while the thermal performance factor is evaluated under an equal pumping power constraint. The experimental results show that the use of the tube fitted with all wire coils leads to an advantage on the basis of heat transfer enhancement over the plain tube with no insert. It is also observed that the uniform-pitch wire coil with higher coil pitch ratio (CR = 8) gives a higher thermal performance factor compared to ones with lower coil pitch ratios (CR = 4 and 6). In addition, for two periodically varying coil pitch ratios, the DI-coil performs with better heat transfer rate than the uniform-pitch ratio (CR = 6) and the D-coil for all Reynolds number ranges studied. The empirical correlations developed in terms of coil pitch ratios (CR), varying coil pitch ratios (D-coil and DI-coil), and Reynolds number are fitting the experimental data within plus or minus 3% and 5% for Nusselt number (Nu) and friction factor (f), respectively. The results of the thermal performance factor for various CR, D-coil, and DI-coil values are also determined.

Effect of hole shape on the heat transfer in a rectangular duct with perforated blockage walls

Heat transfer coefficients were measured by the improved hue detection based liquid crystal technique in a turbine blade internal cooling passage model with blockage walls. In the experiments, blockages with 9 holes of circular, wide, narrow shapes were used and for the circular shape, the number of hole of 7, 9, and 11 were tested. For all cases, the perforated area was kept same. Results showed that the staggered impingement jets increased heat transfer coefficient, however, pressure drop also increased greatly. Generally, Nusselt number ratio and the thermal performance factor decreased as Reynolds number increased. For all Reynolds numbers tested, the blockage wall with wide holes gave more uniform heat transfer coefficient and higher thermal performance factor. As the number of hole increased from 7 to 11, the distribution of heat transfer coefficient became uniform and the thermal performance factor increased.