Experimental Study Of Heat Transfer Research Articles

Experimental studies of turbulent pulsating flow and heat transfer in a serpentine channel with winglike turbulators

Particle Image Velocimetry and Infrared Thermography measurements are applied to explore the effect of quasi-triangular pulsatile flow inlet on active thermal-fluidic performance in a serpentine channel with innovative winglike turbulators. The novel turbulators are mounted in-line on two sidewalls of the channel with the attack angle (α), thickness to chord line ratio (t/C), and pitch to the channel hydraulic diameter ratio (Pi/DH) respectively fixed at 20°, 0.2, and 0.7. The Strouhal number (St) varies from 0 to 0.67 at a fixed Reynolds number (Re) of 10,000. Relative to the steady-state case (St = 0), the flow pulsation not only strengthens the Fujiwhara co-rotating vortices but also eliminates the corner vortex streams, leading to the overall Nusselt number (Nu¯/Nuo) enhancement by 13%. Moreover, both the Nu¯/Nuo and friction factor (f¯/fo) reveal the tilde-like trends with St, resulting in the highest Nu¯/Nuo of 5.2 and thermal performance factor of 1.39 at St = 0.67, which are higher than the previous best results for 10≤f¯/fo≤100 in the Nu¯/Nuo-f¯/fo diagram. Finally, by combing the present data with our previous steady state results, composite correlations of Nu¯/Nuo and f¯/fo versus Re, α, t/C, Pi/DH, and St are proposed.

Experimental study of heat transfer during boiling in a thin layer of liquid on surfaces with structured porous coatings

The paper presents the results of the study of evaporation and boiling in a thin horizontal layer of liquid on microstructured surfaces in a wide range of changes in pressure. It is found that the thermal conductivity of materials of microstructured surfaces significantly affects the mechanism of steam removal from the pores and circulation of liquid along the heat transfer surface. It is determined that the pressure change leads to three regimes of heat transfer: evaporation, transition regime, and bubble boiling. The lowest values of the heat transfer coefficients and CHF were obtained in the transition regime; the highest ones were obtained in the bubble regime on both surfaces. Due to the higher thermal conductivity, the higher heat transfer coefficients and CHF were obtained on the bronze coating than on stainless steel over the entire pressure range.

Experimental study of heat transfer of impinging swirling jets and jets with chevrons

The aim of this work is to study the effect of different forms of passive change in the shape of the flow on the intensity of heat transfer in the impact jet. In this work, a cycle of experiments was performed to investigate an axisymmetric jet flowing normally onto a heated surface. The jet was located both in natural conditions and during swirling of the flow (S = 0.4; 0.7; 1.0). It is shown that the intensity of heat transfer on a heated target in the case of a chevrons jet has little effect on the character, but significantly intensifies heat transfer. In the case of a swirling jet, the intensity distribution on the wall changes its character and locally increases at small distances between the nozzle and the heater.

Experimental and numerical study of heat and mass transfer between high temperature supersonic gas flow and liquid water in a cylindrical duct

The mixing process of a high temperature combustion gas and liquid water conveying large amounts of heat and mass transfer in a very short period of time is investigated experimentally and numerically. A solid propellant produces hot gas into a cylindrical duct where water is supplied through water injectors, facilitating instantaneous mixing accompanied by evaporation and condensation along the duct. Gas generator fire tests are carried out to confirm performance. A cylindrical duct is subsequently connected to the rear end of the gas generator with the liquid water supply system, and experiments are performed for three different amounts of water being injected into the supersonic hot gas. A numerical scheme for a homogenous mixture model with Lee's mass transfer model is applied, and comparisons are made with the experiments. Next, both the experimental and numerical results are fairly compared. The empirical coefficients of evaporation rate for the Hertz-Knudsen-Scharage (HKS) and wall film model (WFM) equations are proposed.

Experimental study of natural convection heat transfer from horizontal cam tubes in a vertical array under constant heat flux

Experimental study of natural convection heat transfer from horizontal cam tubes in a vertical array under constant heat flux

Experimental study of heat transfer in the complex space of small steel containment with a steam jet

Compared with large reactors, the internal structure of small reactors is more compact, and the heat capacity is smaller. The experiments were carried out under various jet velocities, sizes, and horizontal locations to study the heat transfer phenomenon caused by steam jet in a small containment with a complex structure. The results show that the overall temperature distribution in the containment is the same under different jet flow factors. Below the break, the temperature increases with height, and above the break, maintaining the same temperature without thermal stratification. The higher the jet velocity, the higher the overall temperature. As the size of the break increases, the jet velocity decreases, and the circulation in the containment slows down, resulting in the temperature rises. There is no significant difference in temperature field with various jet horizontal locations.

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.