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Highlights • The heat transfer and flow characteristics of two-turn ribbed serpentine channel were analyzed. • Heat transfer distribution is studied with the transient liquid crystal measurement technique. • The inlet Reynolds numbers are varied from 5,000 to 20,000. • Pressure coefficient distribution of two-turn ribbed serpentine channel was researched. Abstract This paper experimentally and numerically investigates the heat transfer and flow characteristics of a two-turn ribbed serpentine channel with lateral outflow. The heat transfer coefficient was measured by a transient liquid crystal technique. Experiments were carried out at Reynolds numbers between 5,000 and 20,000 and rotation numbers of 0 and 0.03. The results indicate that with increasing inlet Reynolds number, the high-Nusselt number regions of the middle and lateral outflow channels gradually move to upstream ribs. The inlet channel shows the highest increase rate of the Nusselt number; whereas, the rate is lowest at downstream turning area. The rotation increases the trailing surface Nusselt number of the inlet and lateral outflow channels, and the increase rate is more prominent for higher values of the Reynolds number. However, the rotation has negative effects on the averaged area Nusselt number of the middle channel in cases of Re ≤ 17,000. The pressure coefficients decrease the in inlet and middle channels along the flow direction; whereas, they slightly increase in the lateral outflow channel. There is a long low-velocity vortex formed in the lateral outflow channel, and the vortex size is considerably reduced by the rotation. Accordingly, the rotation has the most positive effects on the pressure coefficient of the lateral outflow channel. [ABSTRACT FROM AUTHOR]

This paper focused on the trajectory and flow patterns of submerged horizontal gas jet in stagnant ambient liquid. A rigorous mathematical model for the jet trajectory was developed based on geometric analysis, mass conservation, and momentum. Additional, multiple N 2 -Water experiments employing binary image processing were designed and performed. The experimental procedures and mathematical derivations are presented here in details. To assess the ability of the mathematical model to predict the jet trajectory, extensive experimental results were extracted for comparison with the mathematical results. The mathematical results were found to be in good agreement with the experimental results. Furthermore, a dimensionless jet spreading angle model was proposed and fitted to the experiment results. A flow pattern transition criterion based on the penetration depth of the buoyant jet in the water was also discussed, distinguishing the gas buoyant jet form bubbly or transitional flow patterns. [ABSTRACT FROM AUTHOR]

This paper investigated the frost formation on louvered folded fins in microchannel type heat exchangers and the effects of frost accumulation on the time dependent air side convective heat transfer coefficients during transient heat and mass transfer processes. Seven fin geometries, which were commonly used in outdoor microchannel type heat exchangers of air-source heat pump systems, were investigated for a wide range of temperatures and humidity promoting frost growth. The air-side heat transfer coefficient was depended on the rate of frost growth and it sometimes increased during early frost formation and then it decreased sharply. Frost thickness on the fin leading edge, air face velocity, and corresponding heat transfer coefficients and pressure drops were measured and semi-empirical correlations were developed. The frost thickness and air face velocity were key variables for estimating a local time-dependent air-side Reynolds number and for describing the transient behaviour of the local air-side heat transfer coefficient of the fins. The newly developed correlations identified the main parameters that describe frost growth and heat transfer on louvered folded fins under frosting operating conditions and they agree with the experimental data of the present work and from the literature. [ABSTRACT FROM AUTHOR]

This paper reports the hydrodynamic and thermal performance of stainless steel wire mesh blocks in a vertical channel, for different aspect ratios. Commercially available stainless steel wire meshes are arranged side by side to act as a porous block. Convective heat transfer experiments in a vertical channel using an isothermal flat plate are conducted for a Reynolds number range of 60–2380. The plate is sandwiched with wire mesh block on either side so that the assembly completely fills the channel. The pressure drops across the filled region are measured for the velocity range discussed. The plate temperature is measured under steady state conditions. The effect of mesh block thickness on the hydrodynamic and thermal performance is analyzed, keeping the porosity fixed. Plain channel experiments are also conducted under similar conditions for purposes of comparison. The novelty of the present study is that a simple and cost effective method without any permanent bonding is developed for different mesh block thicknesses and its overall performance has been studied. This situation represents the least performance of the porous structure. The performance of the porous blocks is also compared with other porous structures available in literature, based on a performance factor. Nusselt number correlations are developed based on the experimental data. The thermal performance has been compared with brass wire mesh blocks of identical porosity and thickness. [ABSTRACT FROM AUTHOR]

This paper presents the experimental investigation for heat transfer and friction factor for an artificially roughened solar air heater duct having an aspect ratio of 12. Arc shape wire ribs arranged in ‘S’ shape having roughness parameters as relative roughness pitch (p/e) in the range of 4–16, relative roughness height (e/D h ) in the range of 0.022–0.054, arc angle (α) in the range of 30–75°, relative roughness width (W/w) in the range of 1–4 and Reynolds number ( Re ) ranges from 2400 to 20,000. It has been found that performance of roughened solar air heater duct is better than the performance of smooth duct for the range of roughness parameters investigated. Experimental results show that maximum enhancement in Nusselt number ( Nu ) and friction factor (f) have been found at relative roughness width (W/w) value of 3, relative roughness pitch (p/e) value of 8, arc angle (α) value of 60° and relative roughness height (e/D h ) value of 0.043. Based on the data collected from the test runs for roughened duct for various combinations of the roughness parameters correlations are also developed for heat transfer and friction factor in terms of roughness parameters and operating parameter (Reynolds number). [ABSTRACT FROM AUTHOR]

Jet impingement is exceedingly operational for numerous field applications including electronics cooling and process industry. Its implementation comfort makes it common for many applications. The paper deals with investigation of heat transfer characteristics of a cooling flat surface. It is cooled by air jet impinged at an inclination of 15–75° to target plate. For understanding variation in cooling performance, comprehensive experiments are performed with different configurations of inclined jets on hot surface. For simplicity in using air, it is recommended for verity of applications as cooling fluid. Air Inclined jet with Reynolds Number in the range of 2000 ⩽ Re ⩽ 20,000 is examined for the circular Inclined jet. The target to inclined jet perpendicular height ( H ) is varied from 0.5 ⩽ H / D ⩽ 6.8 for understanding effect of H on cooling performance of different locations on target plate. The investigation leads to equations for average Nusselt Number for inclined non-confined air jet for cooling applications. [ABSTRACT FROM AUTHOR]

The paper reports an experimental investigation into the thermal performance and pressure drop characteristics of nanofluid-cooled heat sinks and then compares the results with the data for water-cooled heat sinks. Heat sinks with miniature circular-fin (MCFHS) and square-fin (MSFHS) structures are used and made from aluminum material with dimensions of 28 × 33 mm. Similarly, SiO 2 nanoparticles dispersed in deionized water with particle concentrations of 0.2%, 0.4%, and 0.6% volume are used as working fluids. The effects of pin fin configuration, particle concentration, and flow rate on the heat transfer performance and flow behaviors are presented. Reynolds numbers based on the hydraulic diameter of each flow channel ranging between 700 and 3700, fluid temperature of 15 °C, and heat flux ranging from 2 and 5 W/cm 2 are tested. Hydraulic diameters based on each flow channel are equally designed at 1.2 mm for both heat sinks. The experimental results indicate that the heat transfer coefficient increased with increasing Reynolds numbers and particle concentrations. The MCFHS gave greater heat transfer performance than that of the MSFHS by about 6–9%. For pressure drop data, the measured data showed that the pin fin configuration and particle concentration had small effects on the pressure drop and pumping power. [ABSTRACT FROM AUTHOR]

This paper investigates experimentally the effects of laminar nanofluid flow over the microscale backward-facing step (MBFS) and forward-facing step (MFFS) on the heat transfer characteristics. The experiments were implemented on MBFS and MFFS with a step height of 600 μm. Both MBFS and MFFS have the upstream and downstream lengths of 0.1 m and 0.15 m respectively. The Reynolds number ranged of 280–480. The concentrations of SiO 2 nanoparticle valued at 0.005 and 0.01 with a diameter of 30 nm were immersed in a distilled water. The experimental results revealed that the concentration of 0.01 water–SiO 2 nanofluid recorded the highest Nusselt number. The comparison between MBFS and MFFS revealed that the highest Nusselt number is obtained through the use of the MFFS, which is approximately twice that of MBFS. However, the friction factor recorded a higher value for MFFS. The experimental results were in a good agreement with the numerical published results. [ABSTRACT FROM AUTHOR]

A comparative experimental study on the heat transfer characteristics of steam and air flow in rectangular channels roughened with parallel ribs was conducted with the use of an infrared camera. The effects of Reynolds numbers ( Re ), rib spacing ratio ( P / e ) and rib angles ( α ) on steam and air convective heat transfer were obtained and compared. The blockage ratio ( e / D h ) of the tested channel is 0.078 and the aspect ratio ( W / H ) is fixed at 3.0. In the paper, the Reynolds number for both coolants ranges from 3000 to 15,000, the rib spacing ratios were 8, 10 and 12, and rib angles were 90°, 75°, 60°, and 45°, respectively. For a square channel with 90° ribs with wider rib spacing ratio ( P / e = 10 and 12) respectively give a area-averaged heat transfer about 8.4% and 11.4% more than P / e = 8 model. The heat transfer enhancement of both steam and air increased with decreasing the rib angle from 90° to 45°. Although the heat transfer distributions of steam and air flow in the rib roughness channels were similar to each other in all the investigated channels, the steam flow can obtain a high convective heat transfer enhancement capability. The area-averaged Nusselt numbers of steam flow at a Reynolds number of 12,000 were approximately 13.9%, 14.2%, 19.9%, and 23.9% higher than those of air flow for rib angles of 90°, 75°, 60°, and 45°, respectively. [ABSTRACT FROM AUTHOR]

This paper investigates experimentally the correlation and the effect of the turbulence intensity on the pressure drop and the heat transfer mechanism of a heat exchanger with elliptic tubes in a staggered arrangement. The heat exchanger studied in this work is an air–water cross flow heat exchanger with air being the external fluid and water the internal one. The heat exchanger consists of 144 elliptic tubes placed in a staggered arrangement. The experiments were carried out for two setups. The first setup was referring to isothermal conditions for which only air was used, flowing around the tubes. The second setup was referring to non-isothermal conditions with air as the external fluid and water as the internal working fluid, flowing inside the tubes. The Reynolds number for the external air for the isothermal experiments ranged between 3100 and 5200 based on the maximum velocity between the elliptic tubes. The turbulence intensity values varied between 0.9% and 3%. For the non-isothermal measurements the Reynolds number took values from 3100 to 7700 and the turbulent intensity ranged from 0.9% to 3%. The measurements showed that the increase of turbulent intensity led to a decrease in the total pressure drop of the external flow of the heat exchanger together with an enhancement of the heat transfer mechanism. [ABSTRACT FROM AUTHOR]

• Introduction of air swirling can either enhance or reduce the heat transfer rate depending on operating condition. • Heat transfer enhancement is achieved with the appropriate level of air swirling when the complete combustion occurs in the inner reaction zone. • Impinging inverse flame jet with an appropriate level of air swirling possesses the advantages of both premixed and diffusion flame jets. • Impinging inverse flame jet is a desirable option for impingement heating applications. This paper presents an experimental study to explore the feasibility of utilizing a swirling inverse diffusion flame (IDF) jet for impingement heating. The investigation concentrates on the impinging flame structure and impingement heat transfer. The effects of air swirling, the air jet Reynolds number, Re a , the global equivalence ratio, Φ, and the non-dimensional burner-to-plate distance, H/d a , on the impinging flame shape and impingement heat transfer are investigated experimentally. The Experiments show that introduction of air swirling can either enhance or reduce the heat transfer rate depending on operating condition. Under-swirling produces no evident heat transfer enhancement due to insufficient air/fuel mixing, while over-swirling reduces heat transfer rate because of the dilution of the entrained cold ambient air. Heat transfer enhancement is achieved with the appropriate level of air swirling when the complete combustion occurs in the inner reaction zone around a stoichiometric condition. The investigation shows that the impinging inverse flame jet with an appropriate level of air swirling possesses the advantages of both premixed and diffusion flame jets with blue appearance, ultra-low NO x emission, high flame temperature, good self-stabilization capability, and enhanced heat transfer rate. Therefore, it is a desirable option for impingement heating applications. [ABSTRACT FROM AUTHOR]

• Infrared thermography to resolve transient jet behaviour. • Impinging heat transfer analysis. • Non-swirling (S = 0) and highly swirling jets (S = 1.05). • Correlations presented. A multitude of research has already been undertaken to examine the effects of imparting swirl (S), Reynolds number (Re), and nozzle-to-plate distance (H/D) on the steady-state heat transfer characteristics of turbulent impinging jets. However, no studies to date have compared the transient development of such jets in both swirling and non-swirling conditions. The current paper addresses this gap by using highly resolved (time series) imaged (infrared) data in conditions spanning Re = 11,600, 24,600, and 35,000. The experiments are based on an electrically heated foil (0.025 mm) with jets over S = 0–1.05 and nozzle-to-plate-distances of H/D = 2, 4, and 6. For non-swirling impinging jets, at greater Reynolds numbers, the location of the highest Nusselt number remains fixed for all the time steps in contrast to jets with low Reynolds number. The transient characteristics for H/D = 4 are distinct compared to H/D = 2 and 6, with impingement heat transfer taking more time to reach steady-state for H/D = 4. It is also seen that heat transfer distribution over the impingement plate changes significantly over the small interval of time. For swirling jets, the impingement plate heat transfer characteristics develop simultaneously for the stagnation and wall jet region. The peak Nusselt number shifts to the wall jet region as the intensity of swirl increases. Two correlations (first for non-swirl and low-swirl, later for moderate to high swirl) are proposed to predict the time needed to reach steady-state for Re = 35,000. [ABSTRACT FROM AUTHOR]