Effect of rib spacing on heat transfer and friction in a rectangular channel with 45° angled rib turbulators on one/two walls

An experimental investigation of forced convection heat transfer in a rectangular channel (aspect ratio AR = 5) with angled rib turbulators, inclined at 45°, is presented. The angled ribs were deployed with parallel orientations on one or two surfaces of the channel. The convective fluid was air, and the Reynolds number varied from 9000 to 35,500. The ratio of rib height to hydraulic diameter (e/D) was 0.09, while four rib pitch-to-height ratios (p/e) were studied: 6.66, 10.0, 13.33, and 20.0. The aim of the work was to study the effect of rib spacing on the thermal performance of the ribbed channel. The maps of local heat transfer coefficient in the inter-rib regions have been reconstructed by liquid crystal thermography. The thermal performance of each ribbed channel is identified by the average Nusselt number and by the friction factor. Superior heat transfer performance was found at the optimal rib pitch-to-height ratio of 13.33 for the one-ribbed wall channel and at p/e = 6.66–10 for the two-ribbed wall channel.     

Influence of channel aspect ratio on heat transfer in rotating rectangular ducts with skewed ribs at high rotation numbers

Centerline heat transfer measurements along two opposite ribbed walls in three rotating rectangular ducts roughened by 45° staggered ribs with channel aspect ratios (AR) of 1:1, 2:1 and 4:1 are performed at Reynolds (Re), rotation (Ro) and buoyancy (Bu) numbers in the ranges of 5000–30,000, 0–2, and 0.005–8.879, respectively. These channel geometries are in common use as the internal cooling passages of a gas turbine rotor blade and the tested Ro and Bu ranges are considerably extended from the previous experiences. This study focuses on the heat transfer characteristics in response to the change of AR under the parameter ranges examined. With zero-rotation (Ro = 0), the local Nusselt numbers (Nu₀) along the centerlines of two opposite ribbed walls increase as AR increases due to the increased rib-height to channel-height ratio. The Bu impact on heat transfer appears to be AR dependent, i.e. the increase of Bu elevates Nusselt number ratios Nu/Nu₀ in the square channel but impairs heat transfer in the rectangular channels of AR = 2 and 4. Acting by the Coriolis effect alone, all the leading edge Nu values in the present Ro range are lower than the zero-rotation references but started to recover as Ro increases from 0.1 in the channels of AR = 1, 2 and from 0.3 in the channel of AR = 4. The trailing edge Nu/Nu₀ ratios increase consistently from unity as Ro increases but their responses toward the increase of AR are less systematic than those found along the leading edge. The above findings, with the aids of extended Ro and Bu ranges achieved by this study, serve as the original contributions for this technical community. The Nu/Nu₀ ratios in the rotating channels of AR = 1, 2, and 4 fall in the ranges of 0.6–2.2, 0.5–2.7, and 0.5–2.1, respectively. A set of heat transfer correlations is derived to represent all the heat transfer data in the periodically developed flow regions of three rotating ducts.     

Thermal performance of in-line diamond-shaped pin fins in a rectangular duct

This work experimentally studied the pressure drop and heat transfer of an in-line diamond-shaped pin-fin array in a rectangular duct by using the transient single-blow technique. The variable parameters are the relative longitudinal pitch (X_L = 1.060, 1.414, 1.979) and the relative transverse pitch (X_T = 1.060, 1.414, 1.979). The empirical formula for the heat transfer is suggested. Besides, the optimal inter-fin pitches, X_T = 1.414 and X_L = 1.060, are provided based on the largest heat dissipation under the same pumping power.     

Pressure drop and heat transfer of square pin-fin arrays in in-line and staggered arrangements

This work experimentally studied the pressure drop and heat transfer of a square pin-fin array in a rectangular channel by using the transient single-blow technique. The variable parameters are the relative longitudinal pitch (X_L = 1.5, 2, 2.8), the relative transverse pitch (X_T = 1.5, 2, 2.8) and the arrangement (in-line or staggered). Compared with the open articles, the present relative pitches are smaller and independently variable. The performance of the square pin-fins as the cooling devices is compared with that of the circular pin-fins. Besides, empirical formulas for the pressure loss and the heat transfer are suggested. Finally, the optimal inter-fin pitches are provided based on the largest Nusselt number under the same pumping power, while the optimal inter-fin pitches of square pin-fins are X_T = 2 and X_L = 1.5 for the arrays in in-line arrangements as well as X_T = 1.5 and X_L = 1.5 for the arrays in staggered arrangements.     

Experimental studies on heat transfer and friction factor characteristics of turbulent flow through a circular tube with small pipe inserts

Heat transfer performance and pressure drop tests were performed on a circular tube with small pipe inserts. These inserts with different spacer lengths (S = 100, 142.9 and 200 mm) and arc radii (R = 5, 10 and 15 mm) were tested at Reynolds numbers between 4000 and 18,000. Tap water was used as working fluid. The use of pipe inserts allowed for a high heat transfer coefficient with relatively low flow resistance. The Nusselt number and friction factor increase with the decrease in spacer length. Optimal results were obtained for S = 100 mm (R = 10 mm). Heat transfer rates and friction factors were enhanced by 2.09–2.67 and 1.59–1.85 times, respectively, to those in the plain tube. Performance evaluation criterion (PEC) values were approximately 1.79–2.17. The Nusselt number and friction factor increase with the decrease in arc radius. Small pipe inserts with R = 5 mm and S = 100 mm show maximal heat transfer rates of 2.61–3.33 and friction factors of 1.6–1.8 times those of the empty tube. The PEC values were 2.23–2.7. Compared with other inserts, pipe inserts can transfer more heat for the same pumping power for their unique structure.     

Flow visualizations and heat transfer measurements for a rotating pin-fin heat sink with a circular impinging jet

This work experimentally investigated the fluid flow and heat transfer behaviors of jet impingement onto the rotating heat sink. Air was used as impinging coolant, while the square heat sinks with uniformly in-line arranged 5 × 5 and 9 × 9 pin-fins were employed. The side length (L) of the heat sink equaled 60 mm and was fixed. Variable parameters were the relative length of the heat sink (L/d = 2.222 and 4.615), the relative distance of nozzle-to-fin tip (C/d = 0–11), the jet Reynolds number (Re = 5019–25,096) and the rotational Reynolds number (Re_r = 0–8114). Both flow characteristics of stationary and rotating systems were illustrated by the smoke visualization. Besides, the results of heat transfer indicate that, for a stationary system with a given air flow rate, there was a larger average Nusselt number (Nu₀) for the 9 × 9 pin-fin heat sink with L/d = 4.615 and C/d = 11. For a rotating system, a bigger Re_r meant a more obvious heat transfer enhancement (Nu_Ω/Nu₀) in the case of smaller Re, but Nu_Ω/Nu₀ decreased with increasing Re. In this work, Nu_Ω/Nu₀ in L/d = 2.222 is higher than in L/d = 4.615; among the systems in L/d = 2.222, bigger Nu_Ω/Nu₀ exists in the case of C/d = 9–11, but among the systems in L/d = 4.615, bigger Nu_Ω/Nu₀ exists in the case of C/d = 1–3. Finally, according to the base of Nu_Ω/Nu₀ ? 1.1, the criterion of the substantial rotation was suggested to be Re_r/Re ? 1.154.     

Development of correlations for Nusselt number and friction factor for solar air heater with roughened duct having multi v-shaped with gap rib as artificial roughness

This paper presents the results of an experimental investigation of heat transfer and friction in the flow of air in rectangular ducts having multi v-shaped rib with gap roughness on one broad wall. The investigation encompassed Reynolds number (Re) from 2000 to 20,000, relative gap distance (G_d/L_v) values of 0.24–0.80, relative gap width (g/e) values of 0.5–1.5, relative roughness height (e/D) values of 0.022–0.043, relative roughness pitch (P/e) values of 6–12, relative roughness width ratio (W/w) values of 1–10, angle of attack (α) range of 30°–75°. The optimum values of geometrical parameters of roughness have been obtained and discussed. For Nusselt number (Nu), the maximum enhancement of the order of 6.74 times of the corresponding value of the smooth duct has been obtained, however the friction factor (f) has also been seen to increase by 6.37 times of that of the smooth duct. The rib parameters corresponding to maximum increase in Nu and f were G_d/L_v = 0.69, g/e = 1.0, e/D = 0.043, P/e = 8, W/w = 6 and α = 60°. Based on the experimental data, correlations for Nu and f have been developed as function of roughness parameters of multi v-shaped with gap rib and flow Reynolds number.     

Control of jet impingement heat transfer in crossflow by using a rib

Experimental studies are carried out to investigate the jet impingement heat transfer in crossflow by liquid crystal thermography (LCT). The aim is to assess the possibility of controlling heat transfer by using a rib. The crossflow Reynolds number spans from 80,000 to 160,000 and the velocity ratio ranges from 1.0 to 2.8. The results show that the presence of rib can significantly modify the heat transfer pattern of impinging jet. For all the tested cases, the presence of rib makes the Nusselt number profiles across the stagnation point change from a classical bell-shaped profile to a plateau-like pattern, indicating the enhanced heat transfer region expands more as the rib is present. In particular, the presence of rib has a more pronounced effect on the enhancement of heat transfer at lower velocity ratios (R = 1.0 and R = 1.4). However, in such cases, the local heat transfer in the rib corner region deteriorates. At higher velocity ratios (R = 2.0 and R = 2.8), the presence of rib makes the heat transfer rate more uniform, but meanwhile, it is found that the impinging jet effect tends to be weaker.     

Numerical study of hydrodynamic and heat transfer of nanofluid flow in microchannels containing micromixer

In this study heat transfer and fluid flow of Al₂O₃/water nanofluid in two dimensional parallel plate microchannel without and with micromixers have been investigated for nanoparticle volume fractions of ϕ = 0, ϕ = 4%  and base fluid Reynolds numbers of Re_f = 5, 20, 50. One baffle on the bottom wall and another on the top wall work as a micromixer and heat transfer enhancement device. A single-phase finite difference FORTRAN code using Projection method has been written to solve governing equations with constant wall temperature boundary condition. The effect of various parameters such as nanoparticle volume fraction, base fluid Reynolds number, baffle distance, height and order of arrangement have been studied. Results showed that the presence of baffles and also increasing the Re number and nanoparticle volume fraction increase the local and averaged heat transfer and friction coefficients. Also, the effect of nanoparticle volume fraction on heat transfer coefficient is more than the friction coefficient in most of the cases. It was found that the main mechanism of enhancing heat transfer or mixing is the recirculation zones that are created behind the baffles. The size of these zones increases with Reynolds number and baffle height. The fluid pushing toward the wall by the opposed wall baffle and reattaching of separated flow are the locations of local maximum heat transfer and friction coefficients.     

Heat transfer enhancement of nanofluids in a double pipe heat exchanger with louvered strip inserts

The effect of using louvered strip inserts placed in a circular double pipe heat exchanger on the thermal and flow fields utilizing various types of nanofluids is studied numerically. The continuity, momentum and energy equations are solved by means of a finite volume method (FVM). The top and the bottom walls of the pipe are heated with a uniform heat flux boundary condition. Two different louvered strip insert arrangements (forward and backward) are used in this study with a Reynolds number range of 10,000 to 50,000. The effects of various louvered strip slant angles and pitches are also investigated. Four different types of nanoparticles, Al₂O₃, CuO, SiO₂, and ZnO with different volume fractions in the range of 1% to 4% and different nanoparticle diameters in the range of 20 nm to 50 nm, dispersed in a base fluid (water) are used. The numerical results indicate that the forward louvered strip arrangement can promote the heat transfer by approximately 367% to 411% at the highest slant angle of α = 30° and lowest pitch of S = 30 mm. The maximal skin friction coefficient of the enhanced tube is around 10 times than that of the smooth tube and the value of performance evaluation criterion (PEC) lies in the range of 1.28–1.56. It is found that SiO₂ nanofluid has the highest Nusselt number value, followed by Al₂O₃, ZnO, and CuO while pure water has the lowest Nusselt number. The results show that the Nusselt number increases with decreasing the nanoparticle diameter and it increases slightly with increasing the volume fraction of nanoparticles. The results reveal that there is a slight change in the skin friction coefficient when nanoparticle diameters of SiO₂ nanofluid are varied.     

Pool boiling of R-123/oil mixtures on enhanced tubes having different pore sizes

The effect of enhanced geometry (pore diameter, gap width) is investigated on the pool boiling of R-123/oil mixture for the enhanced tubes having pores with connecting gaps. Tubes having different pore diameters (and corresponding gap widths) are specially made. Significant heat transfer degradation by oil is observed for the present enhanced tubes. At 5% oil concentration, the degradation is 26–49% for T_sat = 4.4 °C. The degradation increases 50–67% for T_sat = 26.7 °C. The heat transfer degradation is significant even with small amount of oil (20–38% degradation at 1% oil concentration for T_sat = 4.4 °C), probably due to the accumulation of oil in sub-tunnels. The pore size (or gap width) has a significant effect on the heat transfer degradation. The maximum degradation is observed for d_p = 0.20 mm tube at T_sat = 4.4 °C, and d_p = 0.23 mm tube at T_sat = 26.7 °C. The minimum degradation is observed for d_p = 0.27 mm tube for both saturation temperatures. It appears that the oil removal is facilitated for the larger pore diameter (along with larger gap) tube. The highest heat transfer coefficient with oil is obtained for d_p = 0.23 mm tube, which yielded the highest heat transfer coefficient for pure R-123. The optimum tube significantly (more than 3 times) outperforms the smooth tube even with oil. The heat transfer degradation increases as the heat flux decreases.     

A porous model for the square pin-fin heat sink situated in a rectangular channel with laminar side-bypass flow

This work illustrates the compact heat sink simulations in forced convection flow with side-bypass effect. Conventionally, the numerical study of the fluid flow and heat transfer in finned heat sinks employs the detailed model that spends a lot of computational time. Therefore, some investigators begin to numerically study such problem by using the compact model (i.e. the porous approach) since the regularly arranged fin array can be set as a porous medium. The computations of the porous approach model will be faster than those of the detailed mode due to the assumption of the volume-averaging technique. This work uses the Brinkman–Forchheimer model for fluid flow and two-equation model for heat transfer. A configuration of in-line square pin-fin heat sink situated in a rectangular channel with fixed height (H = 23.7 mm), various width and two equal-spacing bypass passages beside the heat sink is successfully studied. The pin-fin arrays with various porosities (ε = 0.358–0.750) and numbers of pin-fins (n = 25–81), confined within a square spreader whose side length (L) is 67 mm, are employed. The numerical results suggest that, within the range of present studied parameters (0.358 ? ε ? 0.750, 25 ? n ? 81 and 1 ? W/L ? 5), the pin-fin heat sink with ε = 0.750 and n = 25 is the optimal cooling configuration based on the maximum ratio of Nusselt number to dimensionless pumping power (Nu/(ΔP × Re³)). Besides, based on medium Nu/(ΔP × Re³) value and suitable channel size, W/L = 2–3 is suggested as the better size ratio of channel to heat sink.     

Effect of fin thickness on the air-side performance of wavy fin-and-tube heat exchangers under dehumidifying conditions

This study investigated the effect of fin thickness on the air-side performance of wavy fin-and-tube heat exchangers under dehumidifying conditions. A total of 10 samples were tested with associated fin thickness (δ_f) of 0.115 mm and 0.25 mm, respectively. For a heat exchanger with two rows (N = 2) and fin pitch F_p of 1.41 mm, the effect of fin thickness on the heat transfer coefficient is more pronounced. The heat transfer coefficients for δ_f = 0.25 mm is about 5–50% higher than those for δ_f = 0.115 mm whereas the pressure drop for δ_f = 0.25 mm is about 5–20% higher. The unexpected difference in heat transfer coefficient subject to fin thickness is attributable to better interactions between the directed main flow and the swirled flow caused by the condensate droplet for δ_f = 0.25 mm. The maximum difference in heat transfer coefficients for N = 2 and F_p = 2.54 mm subject to the influence of fin thickness is reduced to about 20%, and there is no difference in heat transfer coefficient when the frontal velocity is above 3 m/s. For N ⩾ 4 and F_p = 2.54 mm, the influence of fin thickness on the heat transfer coefficients diminishes considerably. This is because of the presence of tube row, and the unsteady/vortex shedding feature at the down stream of wavy channel. Based on the present test results, a correlation is proposed to describe the air-side performance for wavy fin configurations, the mean deviations of the proposed heat transfer and friction correlations are 7.9% and 7.7%, respectively.     

New approach relevant to total heat transfer coefficient including the effect of radiation and convection at the ceiling in a cooled ceiling room

In this study, radiative and convective heat transfer coefficients at the ceiling are determined for a cooled ceiling room. Firstly, convective heat transfer is simulated numerically neglecting the radiative heat transfer at the surfaces (ε_f = ε_w = ε_c = 0), then, radiative heat transfer is calculated theoretically for different surface emissivities (ε_f = ε_w = ε_c = 0.5, 0.6, 0.7, 0.8 and 0.9) for different room dimensions (3 × 3 × 3, 4 × 3 × 4 and 6 × 3 × 4 m) and thermal conditions (T_f = 25 °C, T_w = 28–36 °C and T_c = 0–25 °C). Numerical data is compared with the results of correlations based on experimental data given in literature. New equations related to convective and total (including the effect of convection and radiation) heat transfer coefficients for ceiling are found in the current study.     

Effect of rotating cylinder on heat transfer in a square enclosure filled with nanofluids

Convective heat transfer in a differentially heated square enclosure with an inner rotating cylinder is studied theoretically. The free space between the cylinder and the enclosure walls is filled with water–Ag, water–Cu, water–Al₂O₃ or water–TiO₂ nanofluids. The governing equations are formulated for velocity, pressure and temperature formulation and are modeled in COMSOL, a partial differential equation (PDE) solver based on the Galerkin finite element method (GFEM). The governing parameters considered are the solid volume fraction, 0.0 ? ? ? 0.05, the cylinder radius, 0 ? R ? 0.3 and the angular rotational velocity, ?1000 ? Ω ? 1000. The results are presented to show the effect of these parameters on the heat transfer and fluid flow characteristics. It is found that the strength of the flow circulation is much stronger for a higher nanoparticle concentration, a better thermal conductivity value and a smaller cylinder with a faster, negative rotation. The maximum heat transfer are obtained at a high nanoparticle concentration with a good conductivity value, a slow positive rotation and a moderate cylinder size located in the center of the enclosure.     

Natural convection in a rectangular enclosure containing an oval-shaped heat source and filled with Fe3O4/water nanofluid

This work concerns with the study of natural convection heat transfer in rectangular cavities with an inside oval-shaped heat source filled with Fe₃O₄/water nanofluid. The finite element method is employed to solve the governing equations for this problem. Average Nusselt numbers are presented for a wide range of Rayleigh number (10³ ≤ Ra ≤ 10⁵), volume fraction of nanoparticles (0 ≤ ϕ ≤ 14%), and four different size and shapes of the heat source. Depending on concentration of the nanoparticle, geometry of the heat source, and the value of Rayleigh number different behaviors are monitored for average Nusselt numbers. Configuration of the heat source dictates a significant change on the behavior of the average Nusselt number, while addition of the nanoparticles has a negative effect on the magnitude of Nusselt number for this problem.     

Heat transfer enhancement from protruding heat sources using perforated zone between the heat sources

This study is to experimentally investigate the heat transfer enhancement by perforation in air cooling of two in-line rectangular heat sources module. Two separation distances between the heat sources were investigated at s/L = 0.5 and 1.0. The area between the heat sources in both cases were perforated in aligned arrangement such that the holes open area ratio (β) are of 0, 0.0736, 0.1472 and 0.2944. The dimensionless temperature distribution and the average Nusselt number are considered at different values of Reynolds number (3391 ? Re_L ? 10798) and holes open area ratio. It could be seen that perforation could enhance the heat transfer coefficients and reduce the module temperature significantly. Correlations are obtained for the average Nusselt number utilizing the present measurements within the investigated range of the different parameters.     

Experimental investigation of opposing mixed convection hear transfer in the vertical flat channel in a laminar–turbulent transition region

In this paper we present the results on experimental investigation of the local opposing mixed convection heat transfer in the vertical flat channel with symmetrical heating in a laminar–turbulent transition region. The experiments were performed in airflow (p = 0.1–1.0 MPa) in the range of Re from 1.5 × 10³ to 6.6 × 10⁴ and Gr_q up to 1 × 10¹¹ at the limiting condition q_w1 = q_w2 = const. The analysis of the results revealed significant increase in the heat transfer with increasing of air pressure (Gr number). Also sharp increase in heat transfer was noticed in the region with vortex flow in comparison with the turbulent flow region.     

Measurement of local heat/mass transfer coefficients on a dimple using naphthalene sublimation

Local and average heat/mass transfer characteristics on a single dimple were investigated using a naphthalene sublimation technique. The dimple depth in this study ranged from 20% to 40% of the channel height. The experimental conditions covered the range from laminar to low-velocity turbulent flow regimes, 500 ? Re_H ? 5000. Secondary flows from the dimple were clearly observed in the transient flow regime of Re_H = 2000–3000. The velocity fluctuation in the mixing layer over the dimple increased with the dimple depth and the Reynolds number. The impingement of the mixing layer and the induced secondary flows augmented the Sherwood number around the rear rim of the dimple and in the rear plateau region, respectively. For a Reynolds number of 3000, the Sherwood number increased significantly due to the increased fluctuation in the mixing layer and the intensified secondary flows from the dimple. The heat/mass transfer augmentation factors increased as the Reynolds number increased, reaching 1.5 at a Reynolds number of 5000.     

Heat transfer of impinging jet-array over convex-dimpled surface

A detailed heat transfer measurement over a convex-dimpled surface of impinging jet-array with three eccentricities (E/H) between jet-centre and dimple-centre is performed. These surface dimples considerably modify heat transfers from smooth-walled scenarios due to different impinging topologies for jet array with modified inter-jet reactions. Heat transfer variations caused by adjusting jet Reynolds number (Re) and separation distance (S/D_j) over the ranges of 5000 ⩽ Re ⩽ 15,000 and 0.5 ⩽ S/D_j ⩽ 11 with three eccentricities of E/H = 0, 1/4 and 1/2 are examined. A selection of experimental data illustrates the isolated and interactive influences of Re, S/D_j and E/H on local and spatially averaged heat transfers. In conformity with the experimentally revealed heat transfer physics, a regression-type analysis is performed to generate a set of heat transfer correlations, which permit the evaluations of spatially averaged Nusselt numbers over central jet region of dimpled impinging surface.