Experimental Study Of Heat Transfer Research Articles

Experimental and numerical study of upward flame spread and heat transfer over expanded polystyrene at different altitudes

Experimental and numerical study on upward, vertical flame spread was carried out on expanded polystyrene (EPS) at two different altitudes of 50 m and 3658 m to understand the effect that altitude has on flame spread of melted material. The experiments show that, with the altitude increase, the flame spread rate would decrease. At low altitude, the flame spread rate accelerated at later time, while at high altitude, the stagnant stage of pyrolysis front was formed. The numerical simulation could well give the trend of pyrolysis front evolution and the different flame spread behaviors with altitudes. It was found that, at low altitude, Raxincreased slowly in the turbulent period with relatively large entrainment air flow rate, mainly due to the increased and then decreased mass loss rate in the wall fire zone. While, at high altitude, the simulated increasing entrainment air flow rate, combined with the larger preheating length δf and the smaller flame heat flux qf", resulted in the stagnancy of pyrolysis front easily. It was hypothesized that extinction occurred eventually at high altitude due to competition between flame spread rate νf and preheating length δf.

Experimental study of heat transfer and pressure drop analysis of the air/water two-phase flow in a double tube heat exchanger equipped with dual twisted tape turbulator: Simultaneous usage of active and passive methods

In this paper, simultaneous use of active and passive methods for enhancing the thermal performance of a double-tube heat exchanger was investigated. This was done by injecting air bubbles into the working fluid and utilizing a dual twisted tape turbulator. Effects of these two procedures on the Nusselt number and pressure drop were evaluated when applied simultaneously. We further studied the effect of cold water flow rate on thermal parameter. The results indicated that the Nusslte number increases with the decrease of turbulator ratio and increases with the increase of bubble injection flow rate, cold water flow rate and the Reynolds number. The findings further showed that the simultaneous use of the two methods could increase the Nusselt number by 98–114%, 3–14%, and 20–39% compared to the cases with a simple tube heat exchanger, a turbulator-equipped heat exchanger (without bubble injection), and a heat exchanger with bubble injection (without turbulator), respectively.

Experimental study of heat transfer in adsorbed natural gas storage system filled with microporous monolithic active carbon

The study of the thermal state of the monolithic adsorbent layer and internal heat exchange processes during the circulating charging of an adsorbed natural gas storage system was carried out. The correlation between gas flow mode and the heat transfer coefficient between gas and adsorbent is determined under conditions of mass transfer.

Aerodynamics and convective heat transfer processes in cyclone chambers with external recirculation of gases

The paper presents the results of an experimental and theoretical calculation study of aerodynamics and convective heat transfer on the side surface of cyclone recirculation furnace devices. The possibility of controlling the main aerodynamic characteristics without changing the geometric parameters of the cyclone devices by organizing external gas recirculation is shown.

Experimental and computational study of a low-pressure natural circulation loop

The experimental and analytical study of single-phase flow and heat transfer in natural circulation loop has been carried out. Experiments were performed on water and ethanol that are the liquids with significantly different thermophysical properties. Experimental apparatus was a rectangular shaped loop with vertical flow up leg. The flow up and flow down legs of the loop are joined to the separator-condenser at the top of the loop. The upper limit of heat flux densities in the experiments was set with the consideration for flow regime to remain in single phase state along the whole heated length. Wall temperature time records being registered at different distances from the inlet to the heated zone indicate the occurrence of temperature fluctuations near the exit from heated zone even at relatively low heat flux densities. This fact displaces a complex changing of velocity profiles along the tube with vortex formation and occurrence of flow instability. Experimental data on longitudinal wall temperature distributions of heated section have been used to test a modified method of hydraulic calculation of the loop. It was pointed out that in spite of long year (since early 1950s) experimental, analytical and numerical investigations of natural circulation loops no suitable predicting recommendations for heat transfer and friction have been proposed till today for engineering hydraulic calculations of single-phase natural circulation loops.

Heat transfer enhancement during condensation of water steam on inclined pipe

This paper presents experimental study of heat transfer during film condensation of saturated water steam on the outer surface of the inclined pipe by gradient heatmetry. Heat flux per unit area was measured by gradient heat flux sensors made of a single-crystal bismuth. The experimental results are presented in the graphs of heat flux per unit area dependence on time and azimuthal angle. The highest average heat transfer coefficient during condensation of α = 6.94 kW/(m2 • K) was observed when the pipe was inclined at the angle of ψ = 20 °. This value exceeds one obtained on a vertical pipe by 14.9 %. Heat transfer enhancement during condensation of saturated water steam on inclined pipe is associated with changes in condensate film flow. Another part of experiments was made by simultaneously using of gradient heatmetry and condensate flow visualization. Experimental results confirmed the applicability and high informative content of proposed comprehensive method. Comprehensive study of heat transfer during condensation confirmed that heat flux per unit area pulsations may be explained by the formation of individual drops, their coalescence, and drainage from the sensor surface.

Experimental study of propane condensation heat transfer and pressure drop in semicircular channel printed circuit heat exchanger

In this study, the propane condensation heat transfer and pressure drop performance of a printed circuit heat exchanger was experimentally investigated. 30 experimental cases with different mass flux, saturation pressures and vapor quality were conducted. By analyzing these data, a heat transfer correlation similar to the Akers et al. correlation was presented [19]. The newly proposed correlation has a mean square percent error of 5.9% over the equivalent Reynolds number range from 690 to 5600. For the condensing pressure drop correlation, the Lockhart and Martinelli correlation showed the best performance [27].

Analysis of time-dependent heat transfer with periodic excitation in microscale systems

• Analytical, experimental and numerical study of heat transfer on microscale. • Time-dependent temperature distribution with periodic excitation. • Penetration depth at which the periodic-thermal excitations are still noticeable. • A round impinging water jet used to cool a surface heated by pulsing laser. • System reaction for time scales and material properties typical for micro-systems. This study investigates time-dependent heat transfer with periodic excitation in micro-scale systems. Specifically, this study sheds light on time and length scales relevant to periodic heat transfer in micro systems. First, a system's substrate is modeled as a slab of finite thickness, in which the heat conduction equation is solved analytically for a periodic temperature boundary condition over the entire range of transient-periodic process. Using the analytical solutions, the system reaction in time is characterized for time scales and material properties typical for micro-systems. A “penetration depth” is defined as a parameter which indicates the maximum distance from the periodically-heated boundary/surface at which the periodic-thermal excitations are still noticeable. Then, as a case study, an experimental device is examined that uses a round, impinging water jet to cool a surface heated by pulsing laser. Finally, a three-dimensional numerical simulation, validated versus experiments, is used to elucidate the system's expected thermal behavior, including spatial and temporal temperature field variation, relevant time scales for measurements, and the spatial distribution of the heat transfer coefficient. It is demonstrated that the analytical findings can serve to characterize the real behavior rather accurately. The findings can assist in the design of systems with unsteady heating, and in future studies aiming at understanding more complex physically-driven transient phenomena, like flow boiling in micro-systems.

Experimental Study of Unsteady Heat Transfer and Phase Changing Process Upon Impacting of Ice Crystals onto Heated Surfaces Pertinent to Aero-Engine Icing Phenomena

Ice accretion on exposed surfaces of aero-engine components has been widely recognized as a significant hazard to aviation safety in cold weathers. Icing process due to the impingement of the supercooled water droplets suspended in the cloud onto the cold surfaces of inlet components of aeroengines have been studied extensively for decades. Since ice particles were believed to simply bounce off from the exposed surfaces of aero-engine components, ice crystals in the clouds were initially considered not to pose a threat to aviation safety. Therefore, the ice accretion process due to the impacting of ice crystals onto the surfaces of hot engine componentes has not been studied until recently. It has been found recently that, tiny ice particles in the cloud may be partial/full melting upon impacting onto the hot surfaces of aero-engine components, such as heated Inlet Guide Vanes (IGV) and various probes. The partially/fully melted ice crystals were found to stick onto the hot surfaces and form thin water film, which would intercept more oncoming ice particles and lead to significant ice accretion over the surfaces of the hot engine components. The ice crystal induced ice accumulation on the critical aero-engine components has been found to cause significant engine performance loss and erroneous data being read from the probes. In the present study, a series of experimental investigations were conducted to elucidate the underlying physics of the dynamic ice accretion process pertinent to ice crystal icing phenomena. A novel ice crystal icing test rig with the capacity of generating controllable amount of ice crystals and flying speed up to 100 m/s was developed in a temperature-controllable environment chamber for the ice crystal icing studies. By using a high-speed imaging system, a digital particle image velocimetry(PIV), and an Infrared (IR) thermal imaging system, a comprehensive experimental campaign was performed to characterize the transient impacting process of ice crystals, dynamic ice accretion and unsteady heat transfer process associated with the impacting of ice crystals onto heated surfaces, in comparison to those due to the impingement of supercooled water droplets. By using an ultra-sensitive force sensor and a high-speed image system, a comparative study is conducted to examine the differences in the transient impinging dynamics of single water droplets, supercooled water droplets, and ice crystals onto solid surfaces with different wettability and stiffness. By upgrading the unique Icing Research Tunnel of Iowa State University (i.e., ISU-IRT) with additional ice crystal icing capability, a set of explorative studies are also conducted to examine the characteristics of the dynamic ice accretion processes over the heated surfaces of an aero-engine Inlet Guide Vane (IGV) model under both ice crystal icing and supercooled droplet icing conditions. The anti-/de-icing performance of a novel hybrid strategy by integrating icephobic coatings and minimized surface heating are also evaluated under both supercooled water droplet icing and ice crystal icing conditions. The new findings derived from the present studies are very helpful to gain further insights into the ice crystal icing phenomena for the development of more effective and robust anti-/de-icing strategies to ensure safer and more efficient aircraft/aero-engine operations in cold weathers.

Study of Free-Convective Heat Exchange of Air-Coolable Finned Tube Bundles Intensified by Exhaust Shaft

The results of experimental studies of free-convective heat transfer of single-, double- and four-row bundles of finned tubes without and with an exhaust shaft are presented. It is shown that the inter-tube spacing significantly affects the energy efficiency of the bundle. The optimum inter-tube spacing is 61 mm for single-row, about 70 mm for double-row, and over 70 mm for four-row bundles. With increase of the number of rows in the bundle the specific capacity of the bundle increases, but the rate of its increase decreases. The specific heat capacity of a single-row bundle with an inter-tube spacing of 58 mm is higher than with an inter-tube spacing of 64 mm. The specific heat capacity of two- and four-row bundles is less at low exhaust shaft heights, but it is more when the relative shaft height is more than 0.14 and 0.26, respectively. The heat transfer of the bundles can be increased (more than threefold) using optimal ratio of the outlet section of the shaft and the flow capacity of the tube.

Experimental study of heat transfer and friction factor in a double pipe heat exchanger using twisted tape with dimple inserts

ABSTRACT The present experimental work deals with heat transfer and friction factor analysis of water flowing in double pipe heat exchanger with dimpled twisted tape inserts. The investigated parameters are twist ratio of 5.5, Reynolds number (Re) of 6000–14,000, dimple diameter (D) of 3 mm, 5 mm, and 7 mm and dimple diameter-to-depth ratio (D/H) of 1.5, 3, and 6. Nine dimpled twisted tapes were tested and a comparison was made between these twisted tapes and plain twisted tape along with plain tube. Nusselt number (Nu) is found to be maximum (113.68 at Re 14,000 and 59.56 at Re 6000) for D = 5 mm and D/H = 3. This highest value of Nu is 1.50–1.82, 1.37–1.46, 1.24–1.19 and 1.18–1.10 times greater than the values for Nu for plain tube, twisted tape without dimple, dimpled twisted tape with D = 3 mm (D/H = 3) and dimpled twisted tape with D = 7 mm (D/H = 3) respectively. Based on the analysis, the friction factor (f) is found directly proportional to the depth and diameter of the dimple, consequently, maximum value of friction factor (f) is found at 7 mm diameter and D/H 1.5. In addition, it is observed that maximum value of performance evaluation criteria (PEC) is found at dimple diameter of 5 mm (D/H = 3). The correlations for heat transfer and friction factor have been established in terms of dimple geometry (D and D/H) and Reynolds number (Re).

Experimental and Numerical Study of Turbulent Flow and Heat Transfer in a Wedge-shaped Channel with Guiding Pin Fins for Turbine Blade Trailing Edge Cooling

Gas turbine blades employ wedge-shaped channels for the trailing edge internal cooling. Detailed experimental and numerical analyses have been done on the heat transfer and turbulent flow structures in a wedge-shaped channel with arrays of streamlined pin fins for the Reynolds number range from 20,000 to 80,000. The study examines two different configurations of the streamlined guiding pin fins in the wedge-shaped channels, which show the potential of flow guiding and heat transfer improvement for the turbine blade trailing edge internal cooling. The experiments indicate that the streamlined shape of the guiding pin fins causes significantly reduced pressure losses compared to the baseline circular pin fin arrays. Depending on two different configurations the guiding pin fin arrays can achieve pressure loss reduction by up to about 37.3% and 8.7%, respectively, while obtaining appreciably higher average heat transfer on the bottom wall by up to about 8.8% and 12.0%, respectively, compared to the baseline circular pin fin arrays. Besides, the conjugate numerical simulations obtain the total Nusselt numbers of the different pin fin array configurations. Depending on two different configurations the guiding pin fin arrays increase the total Nusselt numbers by up to 11.5% and 13.8%, respectively, compared to the baseline circular pin fin arrays. Additionally, the heat transfer enhancement and pressure loss reduction present more significant promotions for the guiding pin fin arrays as the Reynolds number increases. Also, the numerical simulations reveal the detailed flow structures in the wedge-shaped channels with different pin fin configurations and demonstrate the flow guiding mechanism of the guiding pin fin arrays. Due to the flow optimizations by the guiding pin fin arrays in the wedge-shaped channels, the heat transfer and flow distribution uniformities are improved.

Study of the Convective Stratification of Airflows in a Mine Shaft

The experimental study of heat and mass transfer in the atmosphere of a skip shaft of a Gypsum mine was carried out during the summer and winter seasons. The air flow overturning was observed in the winter season due to the ingress of cold heavy air from the surface into the shaft space. To analyze this phenomenon, a computational fluid dynamics (CFD) model of heat and mass transfer processes in a mine shaft was developed, considering the vertical temperature gradient, roughness of the shaft walls, heat exchange with the shaft lining. Based on numerical simulation, it was found that at relatively low air velocities in the shaft and with a relatively large difference between warm air in the shaft and cold air on the surface, the latter begins to penetrate the shaft and descend down it, gradually filling its space. The interaction of cold air from the surface with the warm air rising up the shaft, as well as with warm walls of the shaft leads to the formation of an unsteady convective vortex cellular structure along the shaft. The depth of the vortex depends on the velocity of the air in the shaft, as well as the difference between the temperatures of the warm and cold air. Based on the obtained numerical simulation data, it was possible to calculate the minimum allowable air velocity at which partial return air flows appear in the shaft cross-section.

Heat transfer by seepage in sand: Influence of saturated hydraulic conductivity and porosity

Heat transfer within the soil is a complex process in the presence of seepage flow. In such conditions, the soil’s thermal behavior is influenced by the thermal and hydraulic properties of the medium as well as the initial conditions and boundary conditions to which the medium is subjected. This paper presents the experimental and numerical studies of heat transfer within the sand subjected to the seepage flow. It focuses on the influence of saturated hydraulic conductivity and the porosity of medium on the heat transfer process. The temperature distribution within the sand was monitored by the optical fiber Distributed Temperature Sensor (DTS). The experiment was performed on three types of silica-dominated sands with different saturated hydraulic conductivities and different Soil Water Characteristic Curve (SWCC). In addition to the experimental study, a coupled hydrothermal numerical model was designed in FEFLOW software and validated by comparing its results with the experimental measurements. To determine the influence of porosity and saturated hydraulic conductivity on heat transfer, we analyzed the numerical models for different values of porosity and saturated hydraulic conductivity. The numerical and experimental studies showed that the thermal velocity is higher in sand with higher saturated hydraulic conductivity and temperature declination occurs more quickly due to the heat convection process. Saturated sand with larger porosity has an overall higher heat capacity, wherefore the temperature declination started later in the measuring points but dropped down lower close to the temperature of the upstream water.

Computational and Experimental Study of Heat Transfer on the heat sink with an impinging synthetic jet under Various Excitation Wave

This work presents an investigation of an impinging synthetic jet produced by a continuous movement of an oscillating piezoelectric membrane in a synthetic jet actuator (SJA). The jet stream blows from the top of the heat sink attached to an electrically heated mat, and is used to enhance heat transfer to simulate a microelectronic component cooling system. Experimental measurements and numerical simulation were conducted to elucidate the distribution of forced convective heat transfer. The membrane's movement was set to produce a synthetic jet for a complete cycle to investigate flow dynamics in suction and blowing streams. The purpose of this research is to characterize synthetic jet membrane vibrations using sinusoidal, square, and triangular wave excitation modes to find which mode is best for vibrating the synthetic jet membrane. The membrane was excited with various functions, including sinusoidal, triangular, and square waveforms with frequency of 80 Hz, 120 Hz, and 160 Hz. The synthetic jet flow simulation utilized a commercial CFD software, FLUENT and DesignModeler to generate an element meshing with total mesh volume of 537.422. The amplitude for all membrane waves is assumed to be 2 mm/s. A k-ω Shear Stress Transport turbulence model was used to complete the simulation. A user-defined function model was developed to drive the piezoelectric membrane using various sinusoidal and non-sinusoidal excitation waves. Measurements were conducted using thermocouples with an automated data acquisition system to obtain temperature data and derive heat transfer coefficients.The conclusion from this research is that the 120 Hz square wave excitation mode produces the best heat transfer value. This research can contribute to the development of synthetic jet models for the microelectronic component cooling system industry.

Experimental study of heat transfer and pressure drop characteristics of microtube condenser using R134a

The condensation heat transfer coefficient and the pressure drop were investigated experimentally and numerically in a microtube condenser using R134a. The microtube condenser comprises of trapezoidal oblique finned microchannel at the top and rectangular fins at the bottom of the tube. Water and air are used as cooling fluids on both sides of the microtube condenser. The experimental setup is validated initially with the existing work of Shah. The condensations effect included the vapour quality and mass flux ranging from 0.25 kg/m2s to 0.9, 99 to 468 kg/m2s, respectively. The results indicate that the condensation heat transfer coefficient and pressure drop enhance with mass flux and vapour quality. The experimental and numerical results are compared with existing experimental work. The microtube condenser enhances the heat transfer coefficient to 22.4% and a pressure drop of 5.6% more than the existing model. The heat transfer coefficient and pressure drop obtained from the numerical study is in superior agreement with the experimental results.

Experimental and numerical study of heat transfer and flow characteristics with different placement of the multi-deck display cabinet in supermarket

Abstract This work focuses on the heat transfer and flow characteristics with the different placement of the multi-deck display cabinet and tries to optimize the placement position of refrigerated display cabinet. First, the temperature distribution in a refrigerated display cabinet was experimentally investigated. The result showed that the food temperature in front is 3.6–4.8°C higher than back row of the same layer, and temperature fluctuation of 0.3–0.7°C less than the back row. Then, a three-dimensional numerical model of the display cabinet was established, and the k–ε model is employed to compare and analyze the heat transfer and air curtain characteristics. The results show that the placement methods have great influence on the performance of the display cabinet. The face-to-back placement method can acquire a better food refrigeration performance, and the food temperature of the face-to-back placement method is 0.3–0.5°C lower than that of the face-to-face placement method.

Experimental study of heat transfer inside a real scale innovative air solar collector

The study is performed on a glazed transpired solar collector, investigating the effect of different distances between the glazing and the absorber (30, 50, 70, and 100 mm) for different airflow values. The results show that the best solution among the tested configurations is with 30 mm distance between the glazing and absorber, no matter what the air flow rate is. The efficiency of the GTC in this case is 16-19% higher for air flow rates around 100 m3/h, m2 and even 30-40% higher for air flow rates around 150-200 m3/h, m2 – compared to configurations with larger distance between the glazing and absorber.

An experimental study of forced convective heat transfer for fully developed Al2O3–water nanofluid in an annulus tube

AbstractNanofluids have been known as practical materials to ameliorate heat transfer within diverse industrial systems. The current work presents an empirical study on forced convection effects of Al2O3–water nanofluid within an annulus tube. A laminar flow regime has been considered to perform the experiment in high Reynolds number range using several concentrations of nanofluid. Also, the boundary conditions include a constant uniform heat flux applied on the outer shell and an adiabatic condition to the inner tube. Nanofluid particle is visualized with transmission electron microscopy to figure out the nanofluid particles. Additionally, the pressure drop is obtained by measuring the inlet and outlet pressure with respect to the ambient condition. The experimental results showed that adding nanoparticles to the base fluid will increase the heat transfer coefficient (HTC) and average Nusselt number. In addition, by increasing viscosity effects at maximum Reynolds number of 1140 and increasing nanofluid concentration from 1% to 4% (maximum performance at 4%), HTC increases by 18%.

Heat and mass transfer in the blasted layer of vegetable materials with cyclic microwave power supply

The results of modeling and experimental study of heat and mass transfer in a dense blown layer of plant materials under cyclic microwave exposure are presented. The two-dimensional mathematical model consists of the equations of conservation of the mass of the gas phase, filtration, heat and mass transfer in phases, which take into account the internal resistance to heat and moisture transfer in the particles when determining the heat and mass transfer coefficients. In this case, the dependences of the heat of the phase transition on the humidity of the particles, their shrinkage during dehydration, and the dependences of the effective coefficients of thermal conductivity of the gas and vapor diffusion on the filtration rate are taken into account. The simulation results of drying chopped potatoes in a dense layer with a cyclic microwave convective energy supply are presented. The possibility of intensifying the process of moisture dehydration and reducing its duration compared with the convective method is shown. Comparison of calculated data with experimental data confirms the adequacy of the model.