Experimental Heat Transfer Research Articles

Pool boiling enhancement and a method of bubble diameter determination on surfaces with deep minichannels

Pool boiling heat transfer experiments were conducted with ethanol, Novec-649 and FC-72 at atmospheric pressure on surfaces with deep minichannels. The minichannels 5.5, 6.0 and 10 mm deep and 0.5–1.2 mm wide were uniformly spaced with a pitch of 2.0 mm on the base surfaces. Heat transfer coefficients obtained with ethanol were similar for all surfaces tested. Novec-649 and FC-72 produced the highest coefficients at heat fluxes above 100 kW/m2 in 1 mm wide minichannels. All the working fluids provided a significant increase in the maximum (critical) heat flux. The images were recorded using high speed imaging techniques across the entire surface of the samples. Boiling visualization aimed to study vapor bubble diameter changes during the growth period and measure the bubble departure diameter.The bubble diameter at departure was also determined analytically according to force balance static and dynamic variants. The dynamic variant provided a high consistency between the theoretical and experimental data for bubble departure diameter with FC-72 and Novec-649.

Nanodiamond Colloids heat transfer behavior in electronics thermal management – an experimental study

ABSTRACT This article reports the heat transfer data obtained from experimenting a novel functionalized nanodiamond (fND) colloid. In this functionalization technique, the nanodiamonds are coupled with molecules of the host fluid via carboxylic bonds, leading to a de-aggregated and fully stable nanodiamond colloid. The colloid flows through a conduction cold plate where the system cools the electronic component, leading to the resultant heat dissipation. This surface modification on the fully functionalized nanodiamonds ensures ultimate stability of the colloidal suspension of nanodiamond particles. The heat transfer experiments were performed with different concentrations of nanodiamond (0.05, 0.10, and 0.20 wt.%) and under a turbulent flow regime (6,400 < Re < 17,000). The closed-loop heat transfer apparatus utilizes a pump equipped with a variable frequency drive to study the pumping power at various flow rates and fND concentrations. The experimental results of this paper show the remarkable effect of low-concentrated fND additives on enhancing the heat-transfer coefficient of the deionized water. The results show up to a 70% enhancement in the heat transfer coefficient compared to the base fluid at the same pumping power using a 0.20 wt.% fND. The nanodiamond colloid has shown no sedimentation after remaining stored over two years after synthesis.

Preliminary development and validation of ACENA code for heavy liquid metal-gas two phase flow simulation

To simulate the heavy liquid metal-gas (HLM-gas) two phase flow phenomena that may occur during postulated accidents of Lead-based fast reactors (LFRs), a 2D multiphase fluid dynamics code, ACENA, is preliminarily developed and validated in this study. A brief description of theoretical models, including governing equations, auxiliary models and material properties are listed in this study. The capability of simulating flow and heat transfer characteristics between gas and liquid phases has been examined by typical air-water bubble column experiment, LBE-N2 two phase flow experiment and Lead-Ar heat transfer experiment, and the comparison results show that this code could attain acceptable accuracy in simulating flow and heat transfer characteristics of HLM-gas two phase flow with appropriate interfacial drag coefficient and heat transfer coefficient models.

Experiments on heat and mass transfer in enthalpy exchangers having a diagonal configuration

ABSTRACT In this study, with an aim to develop a proper effectiveness-NTU relationship of an enthalpy exchanger having a diagonal flow configuration, heat and moisture transfer experiments were conducted. Test samples having a range of diagonal angles were tested, and the effectiveness ratios between the diagonal flow and the cross-flow configuration were obtained as a function of the diagonal angle. Developed correlations of the heat, moisture, and enthalpy predicted the data within 3%. Furthermore, it was shown that the moisture transfer effectiveness was highly dependent on the relative humidity and the air temperature.

Relationship Between Pressure Drop and Heat Transfer of Fully Developed Flow in Smooth Horizontal Circular Tubes and Spiral-Coiled Tubes in the Laminar Flow Regime

Abstract Studies on isothermal steady-state frictional pressure drop for flow of petroleum base oils SN70, SN150, diesel, and water are carried out in spiral coils with diameter to length ratio, 0.00042, 0.00047, 0.00073, 0.00164, 0.00189, 0.003, and 0.0037. An attempt is made to correlate friction factors with a better and more appropriate dimensionless group for flow of Newtonian fluids through spiral-coiled tubes. An innovative approach of correlating heat transfer data with the newly established dimensionless group is presented. Heat transfer experiments are performed for spiral coils with diameter to length ratio 0.000474, 0.00042, 0.001896, 0.00198, 0.000942, and 0.00164 in laminar flow regime. Suitable correlations for friction factors and Nusselt numbers are proposed. Relationship between pressure drop and heat transfer is studied. The incapability of the conventional analogy equations to estimate the heat and momentum transfer coefficients for laminar flow through straight or curved tubes is explained based on the viscous and form drag existing in straight and curved pipe flow. The limitations of the existing analogy equations are examined critically. A new general analogy equation is derived for laminar flow through spiral and straight tubes considering the influencing geometrical parameters of the tube.

Investigation on the temperature distribution in the two-phase spider netted microchannel network heat sink with non-uniform heat flux

The considerable increase of the heat flux and chips density on the phased array antenna has brought great challenges to the thermal management. The objective of the work is to explore an effective scheme to achieve ideal uniform temperature distribution and lower maximum temperature with the condition of non-uniform heat flux. Taking into account of the high efficiency of microchannel heat sink and the promising performance of the two-phase flow boiling heat transfer due to its merit in keeping uniform surface temperature, a novel two-phase spider netted microchannel network heat sink is proposed in this paper. Firstly, a three-dimensional fluid-solid-thermal conjugate model is developed to investigate the flow boiling heat transfer performance of the spider-netted microchannel network numerically. During the numerical calculation, a multi-scale meshing generation method is proposed to reduce modeling difficulties on the premise of proper mesh quality. Then, the flow boiling heat transfer experiment is conducted to verify the reasonable of the model and the simulation results. Finally, the flow boiling heat transfer performance in the spider netted microchannel network is compared to the typical straight microchannel network. Results show that the proposed structure yields better flow boiling heat transfer performance with lower thermal resistance and more satisfactory temperature uniformity mainly due to its uniform flow distribution and enhanced vapor movement. It is also found that the temperature uniformity has reached 1.58 °C at heat flux of 100 W/cm2, but it decreases with the increase of heat flux. This situation cannot be effectively solved by means of increasing inlet flow rate. Thus the structure optimization is conducted to enhance the flow boiling heat transfer performance further, and the optimal structure can meet requirements of temperature uniformity within 2 °C at higher heat flux of 150 W/cm2.

The Effect of Mass Flow Rate and Heat Flux on Vertical Two-Phase Flow Regimes in a Small Diameter Tube

In recent years, there has been a rising concentration in investigating and researching the boiling of micro channel flow; since they afford high thermal effectiveness, small size, and low weight. In this paper, the boiling heat transfer was experimentally examined using water as the working fluid in small diameter tubes. Experiments for the flow model were performed using the same Pyrex glass tube test facility. The experiments of heat transfer were carried out using an internal diameter copper tube (10.922 mm). The range of other parameters was varied: mass flow rate (0.008–0.0214kg/s); heat flux (17351–80529W/m2); it has been found that the average coefficient of heat transfer depends on the form of flow pattern. The average heat transfer coefficient increases by about 37% when the flow pattern varied from slug flow to churn flow at a constant mass flow rate of 0.008kg/s. Besides, the heat transfer coefficient (h) in the two-phase flow, is 39%, which is more than that for single-phase flow, when the mass flow rate is 0.0214kg/s, and heat flux) increases from 17351W/m2 to 49105W/m2. Also, when mass flow rate rises from (0.008kg/s) to (0.0166kg/s), the pressure drop increases by about (15%) and increases by about (5%) when the mass flow rate rises from (0.0166kg/s) to (0.0214kg/s). The main regime of flow was a slug, churn, bubbly, dispersed bubbly, and confined bubbly flow.

Investigation of convective heat transfer and flow hydrodynamics in rectangular grooved channels

An experimental and numerical investigation is conducted for convective heat transfer and flow characteristics in a channel with rectangular grooved top and bottom walls. The study is performed between 2 × 103and 6.5 × 103 of Reynolds numbers. Heat transfer experiments are conducted by applying constant heat flux to the channel upper and lower walls. The averaged Nusselt number, Nu, is presented as an indicator of heat transfer. In the experiments, 1.9–2.4 times larger Nu values were obtained by using grooved channels instead of straight channels. Pressure measurements show that the corrugated channel causes an increase in pressure loss. The Particle Image Velocimetry, PIV method is employed to reveal the flow hydrodynamics and its relation with convective heat transfer. Using the ability of the PIV method to provide instantaneous flow data, interactions between the recirculation flow bubbles in the grooves and the mainstream are observed. In the numerical part of the study, the k-ε turbulence model is used, and predicted results are found to be consistent with experiments. The validated numerical method is used to investigate the effect of groove aspect ratio on heat transfer and pressure drop.

Flow blockage analysis for fuel assembly in a lead-based fast reactor

Flow blockage of the fuel assembly in the lead-based fast reactor (LFR) may produce critical local spots, which will result in cladding failure and threaten reactor safety. In this study, the flow blockage characteristics were analyzed with the sub-channel analysis method, and the circumferentially-varied method was employed for considering the non-uniform distribution of circumferential temperature. The developed sub-channel analysis code SACOS-PB was validated by a heat transfer experiment in a blocked 19-rod bundle cooled by lead-bismuth eutectic. The deviations between the predicted coolant temperature and experimental values are within ±5%, including small and large flow blockage scenarios. And the temperature distributions of the fuel rod could be better simulated by the circumferentially-varied method for the small blockage scenario. Based on the validated code, the analysis of blockage characteristics was conducted. It could be seen from the temperature and flow distributions that a large blockage accident is more destructive compared with a small one. The sensitivity analysis shows that the closer the blockage location is to the exit, the more dangerous the accident is. Similarly, a larger blockage length will lead to a more serious case. And a higher exit temperature will be generated resulting from a higher peak coolant temperature of the blocked region. This work could provide a reference for the future design and development of the LFR.

Spot conductance measurement of aluminum alloys at cryo-temperatures under varying pressure conditions

Thermal contact conductance (TCC) is a key parameter, which influences the reliability of various cryogenic systems. The accurate estimation of TCC is a basic requirement for the optimal design of cryogen-free conduction cooling systems. The lack of reliable TCC results for bare demountable aluminium contacts in the sub-ambient temperature range (50–300 K) prompted us to undertake the systematic experimental investigations. A unique experimental setup has been fabricated to carryout axial heat transfer experiments, which is used for the estimation of TCC between aluminum contacts in the temperature range of 50–300 K. The influence of changing interfacial temperature (50–300 K), interfacial pressure (0.5–12 MPa), and surface roughness parameters (σ = 1.51 to 11.06 μm), as well as the surface finishing process on TCC have been investigated systematically. The finite element (FE) rough surface contact analysis has also been carried out, and real contact area and the TCC has been estimated over the range of interfacial pressure and temperature under considerations. The real contact area was found to be less than 0.1% of the nominal area, even at 12 MPa, and it varies almost linearly with temperature and pressure. The FE estimates of TCC has shown a good agreement with experimental TCC results. Finally, an empirical correlation of TCC has been proposed by incorporating three of the functional parameters, while considering temperature-dependent thermo-mechanical properties into the analysis.

Forced Convection Teat Transfer from a Cross Circular Yawed Tube.(Dept.M)

Complementary heat transfer and flow visualization experiments were performed lo investigate the effect of yaw on tube. Wind tunnel experiments encompassing seven angles of yaw between ᶱ= o( crossflow ) and 28° yielded Nusselt numbers extending over the range of free stream Reynolds numbers from 9000 to 25000. It was found that as the cylinder was yawed with respect to the crossflow orientation, the Nusselt number at first increased, attained a maximum at a yaw angle o 5o and then decreased to a broad minimum which occurred in the range of yaw angles between 20 and 25o and then start to somewhat increase at yaw angle equal 28o. As a further supplement to the heat transfer experiments, the pattern of fluid flow adjacent to the cylinder surface was visualized by means of the oil lampblack technique. The results were compared with the available experimental data. The values of the average Nusselt numbers were fitted in a correlation. This correlation indicates that the Nusselt number is a significant function of Reynolds number and angle of yaw.

Heat transfer performance of steam/air flow in inverted V-shaped rib-roughened channels

The heat transfer performance of steam and air flow in a rough rectangular channel with different inverted V-shaped ribs was investigated by infrared thermal imaging technology. Under the conditions that the Reynolds number is in the range of 4000–15,000, the effects of the rib angle on the heat transfer enhancement of the two coolants were obtained. The rib pitch ratio of the flow channel is 10, the ratio of the rib height to the channel hydraulic diameter is 0.078, and the inverted V-shaped rib angle varies from 45° to 90°. The results show that in the inverted V-shaped ribbed channel, the Nu number on both sides of the channel is greatly increased, while the Nu number in the middle of the channel is lower. The local Nu distribution on the surface of the ribbed channel is highly related to the shape of the rib. For different medium cooling, the value and unevenness of the heat transfer coefficient are different, but the shape of the high and low heat transfer coefficient distribution is hardly affected. The heat transfer of both coolants increases as the rib angle decreases from 90° to 45°. Compared with air flow, steam flow cooling shows higher convective heat transfer enhancement. For rib angles of 45°, 60°, 75°, and 90°, under the operating condition of the Reynolds number = 12,000, the area-averaged Nusselt numbers of the steam flow is 23.6%, 27.4% and 13.9% higher than that of the air flow, respectively. Based on the experimental heat transfer data, the correlation in terms of the Reynolds number and the rib angle was developed, which is used to estimate the Nu number for steam and air cooling in the inverted V-shaped rib-roughness channels.

Theoretical and experimental analysis of overall cooling effectiveness for afterburner double-wall heat shield

The porous heat shield installed in the afterburner can effectively prevent the afterburner cylinder from overheating. Impingement/effusion cooling is one composite cooling technique to effectively improve the overall cooling effectiveness of the heat shield. Designing effective and efficient double-wall configurations requires a detailed understanding of how geometry and operating conditions affect the way coolant cools the afterburner cylinder. In this study, conjugated heat transfer experiments were conducted using IR thermography for double-wall heat shields with three open areas of the impingement plate (σ = 0.5%, 0.7%, 0.8%). The momentum flux ratio was varied in the range 0.01≤I≤0.50. The mainstream Reynolds number, based on the diameter of the film hole, was varied in the range 800≤Reg≤2000. The effects of geometrical and aerodynamic parameters on overall cooling effectiveness were experimentally and numerically investigated with the comprehensive effects of solid heat conduction and convection heat transfer. The experiments were modeled in a low-temperature state (Tg = 603 K, Tc = 298 K), which was based on the analogy theory and matching principle, as if the experiments were conducted under engine operating conditions. The modeling accuracy of experimental results was analyzed by the analogy theory. Results show that the difference in the overall cooling effectiveness between “laboratory” conditions (Tg/Tc = 2) and “engine” conditions (Tg/Tc = 4) is within 0.048. The overall cooling effectiveness is mainly affected by the open area through adiabatic cooling effectiveness and internal heat transfer, and that is mainly affected by the mainstream Reynolds number through external heat transfer. According to the area-averaged results in given experimental conditions, the maximum overall cooling effectiveness raise and drop coefficients after increasing open area and mainstream Reynolds number are 17.3% and 6%, respectively. Thus, the high open area and low mainstream Reynolds number are beneficial to improve the overall cooling effectiveness of the afterburner double-wall heat shield. Furthermore, the experimental measurements provide an important database for the evaluation of computational fluid dynamics simulations of the conjugate heat transfer effects occurring in the double-wall heat shield.

Computational fluid dynamics analysis of buoyancy-aided turbulent mixed convection inside a heated vertical rectangular duct

Heat transfer in buoyancy-aided turbulent mixed convection inside a vertical rectangular duct with air was investigated through steady-state computational fluid dynamics (CFD) analysis with Reynolds-averaged Navier–Stokes turbulence models. Heat transfer can occur in the passive safety system of a nuclear reactor, which uses natural circulation as an initial driving force owing to its relatively low flow rate and strong heat flux condition. One example is the reactor cavity cooling system in a very high temperature gas-cooled nuclear reactor, whose high operating temperature enables high-efficiency power generation and hydrogen production. In buoyancy-aided turbulent mixed convection, heat transfer can deteriorate under specific conditions. Therefore, heat transfer must be analyzed because its deterioration can affect the performance of the safety system, which has an influence on the safety of the nuclear reactor. In this study, the results of a CFD analysis were compared with experimental results obtained from the riser heat transfer experimental facility, which is an experimental heat transfer facility with a 4.0 vertical rectangular duct test section. The CFD analyses were performed using a realizable k–ε model and a v2–f model, and the results were compared with the experimental results. It was confirmed that the results of the v2–f model exhibited good agreement with the experimental results. Based on this fact, four additional idealized cases were calculated using the v2–f model to investigate heat transfer deterioration in the turbulent mixed-convection regime. The calculation results revealed that the gradient of the velocity profile from the wall toward the center was reduced outside the near-wall region owing to buoyancy, which led to a reduction in the shear stress, turbulence generation, and heat transfer. In addition, the velocity gradient beyond the near-wall region reached a negative value above a certain buoyancy number, and turbulence generation and heat transfer started to recover. This analysis of heat transfer deterioration can provide insight into the construction of a heat-transfer coefficient correlation for buoyancy-aided turbulent mixed convection, and it will contribute to the development of heat transfer predictions and the evaluation of safety systems in nuclear reactors.

The Effect of Tripping Wire Rings on The average Heat Transfer from a Cross Circular Cylinder.(Dept.M)

Forced convection heat transfer experiments were provided to study the effects of tripping wire rings on the average heat transfer from a cross circular cylinder. The experimental data have been obtained When the test tube is covered by a number of tripping wire rings as well as the results obtained when the test tube is bare. The number of the tripping wire rings ranges between 0 and 53. The results have been presented in terms of Nusselt number and Reynolds number as well as the pitch to-diameter ratio of the tripping wire rings. Wind tunnel experiments extended over the range of the frec stream Reynolds numbers from 7.19x103 to 5.97x104. The pitch-to-diameter ratio varies between 0.172 and 1.131. The experiments are carried out for a constant rate of heat flux. The values of the average Nusselt number are fitted in a correlation. The correlation indicates that the Nusselt number is a significant function of Reynolds number and pitch to-diameter ratio. The augmentation of heat transfer varies from 13% to 62% as the pitch to diameter ratio varies from 1.131 to 0.172.

Effect of bend radius and insulation on adiabatic section on the performance of a single closed loop pulsating heat pipe: experimental study and heat transfer correlation

Pulsating heat pipes are a relatively newer generation of heat pipe which are wickless thus more economical and less fabrication-intensive. These devices are made of a capillary sized tube bent in a serpentine form to result in a closed system. In this study a single closed loop pulsating heat pipe made of 2 mm internal diameter copper tubing is experimentally investigated for two different bend radii. The performance comparison is made for two fluids – methanol and water, for varying heat loads from 10 W to 100 W, with various inclinations from vertical bottom heated mode to horizontal mode, for a constant fill ratio of 60%. The sharper bend, though making the device more compact, is found to reduce the performance. One of the bend geometries is further investigated for the effect of insulation on the adiabatic sections. The insulation is found to certainly widen the heat load range though a distinct performance improvement is not found. The variation of the frequency of pressure pulsations with respect to the fluid, heat load and inclination is also studied. Additionally, one of the geometry is tested with FC 72 fluid as well to arrive at a correlation for heat flux for varying fluids, heat fluxes and inclinations, covering 116 data points. The performance trends predicted by the correlation are in line with those reported in the literature.

A Comprehensive Heat Transfer Study Inside a Steam Turbine Valve: Experiment, Numerical Simulation, and Simplified Model

Abstract In a steam turbine system, one of the main factors limiting the operational flexibility is the thermal stress associated with a high-temperature gradient within the control valves, which often leads to structural damage during frequent startup and shutdown cycles. One possible solution is to utilize an electric heating system with appropriate insulation to decrease the warm-up time. Here, an experiment and a numerical simulation were performed using a scaled turbine valve equipped with an electric heating system to understand the heat transfer process. The experiment, which was conducted at Shanghai Jiao Tong University, had a duration of 100 h and included three heating–cooling cycles and two heat preservation states. The results showed that the temperature distribution of the scaled valve was uniform, which is due to the high thermal conductivity of the valve body and the low conductivity of the insulation layer. The simulation, which used the commercial software ansysfluent 2019 R1, was performed to model the experimental heat transfer process and showed less than 10% deviation from the measured temperatures. A sensitivity study revealed that the valve temperature is very insensitive to the heat transfer coefficient of the outer surface of the insulation layer, which is attributed to the negative feedback regulation of radiation heat transfer. To further improve the computing efficiency, a simplified model based on the lumped parameter method and the quasi-steady-state assumption was proposed and validated. This model can predict the 100 h valve temperature history in less than 1 min and showed good agreement for all of the studied cases. The ability of the simplified model to simulate the valve heating–cooling cycles at a high efficiency could accelerate the thermal design process to improve the operational flexibility of steam turbines in the future.

Systematic micro heat sink optimization based on hydrofoil shape pin fins

Multi-objective optimization of the hydrofoil shaped pin fins for micro heat sinks was studied. Optimization tool utilized a multi objective genetic algorithm (MOGA) and a numerical model that was validated against previously obtained experimental single-phase heat transfer results on a hydrofoil shape micro scale pin fin. The numerical model was built using StarCCM + software. Several turbulence models were examined against single-phase experimental data and the Lag-EB k-epsilon turbulence model was chosen for validation. Optimization tool varied the shape of the hydrofoil pin fin through flexible parametrization utilizing an ellipse and polynomials. An algorithm coupling pin fin parametrization code, validated the 3D CFD solutions and a MOGA interface was implemented. Subsequently, multi-objective optimization was performed to maximize the heat transfer coefficient while minimizing pressure drop. A broad range of novel geometries were obtained with a tradeoff between thermal and hydraulic performance for low Reynolds number. Novel pin fin shape, bird design, is proposed as a result of the study. Additional 3D steady state conjugate CFD simulations including the bird design pin fin were performed using already verified setup, and it was shown that the enhancement in the average heat transfer coefficient dominates over the pressure drop increase at higher Reynolds numbers.

Evaluation of conventional invasive measurements and examination of non-invasive measurement technique on human body core temperature

It is important in many cases to measure and monitor, human body core temperatures, to prevent the likes of heat stroke and hypothermia. Measuring core body temperatures is also important for example, in relation to the basal body temperatures of women, and for improving the quality of life for vulnerable people such as infants, and those with cervical spine injuries. In doing so, thermal environmental changes can be monitored. However, todays conventional measuring method is apparently invasive because a temperature sensor has to be inserted into the body, from the outside. Characteristics of the body parts measured were considered by the subject experiments. There was a difference in core temperatures, depending on the measurement body parts, and it was found that the temperature decreased in the order of rectum, sublingual, and tympanic during normal times. It was confirmed that the tympanic temperature showed the most significant increase in core temperature, with running and the sublingual temperature the best in responsiveness. As a non-invasive core temperature measurement method, the basic characteristics were examined by the heat transfer experiment for the dual-heat-flux method. Thus, it was clarified that adequate temperature accuracy can be guaranteed if appropriate materials, thicknesses, and sizes are selected and the adiabatic condition for the peripheral part is fulfilled.

The experimental thermal and hydraulic performance analyses for the location of perforations and dimples on the twisted tapes in twisted tape inserted tube

Experimental heat transfer enhancement investigation is carried out by using various twisted tape inserted tube configurations under turbulent flow conditions. Constant heat flux is applied on the outer wall of the test tube. The working fluid is selected as water. The twisted tapes having twist ratio (y/w) of 5.88 are modified with perforations (PTTE) and dimples (DTTE) at the edge of the twisted tape. The ratio of the distances between the perforations/dimples (p) to the twist length (y), i.e. pitch ratio (p/y), are selected as 0.25, 0.50 and 1.00. The results show that using twisted tapes lead to increase thermal performance when compared to the smooth tube. The DTTEs promise more heat transfer performance rather than the PTTEs, while the PTTEs gives less friction loss penalty than the DTTEs. Overall results are given as performance evaluation criteria and the highest PEC value is obtained as 1.57 for DTTE_p/y = 0.25 type twisted tape insert used case at Reynolds number of 6367. In addition, the paper includes a comparative evaluation in terms of the heat transfer and the hydraulic performance about the perforations/dimples at the edge or centre of the twisted tape.