Experimental Heat Transfer Research Articles

Visualization and Heat Transfer Performance of Mini-Grooved Flat Heat Pipe Filled with Different Working Fluids.

Mini-grooved flat heat pipe (MGFHP) possesses the advantages of high compactness, no mechanical component, super thermal conductivity, and excellent temperature uniformity, which can meet the demand for electronic devices efficiently cooling. In this research, visual and heat transfer experiments were performed to investigate the flow and thermal characteristics inside the MGFHP. Fluid flow and distribution are observed to be quite different in the MGFHP containing different working fluids, which is affected by the physical properties of working fluid, the surface state of the grooved wick, and limited working space. Additionally, the input heat, working fluid type, filling ratio, and wettability obviously affect the thermal conductivity and temperature uniformity of the MGFHP. The deionized water-filled MGFHP possesses lower thermal resistance and higher heat transfer capacity than anhydrous ethanol or hexane filled MGFHP, especially for the copper oxide MGFHP filled with deionized water with a filling ratio of 1.0. Thermal resistance, maximum temperature, and temperature nonuniformity at the condensation section of deionized water-filled copper oxide MGFHP are lower than those of the original copper MGFHP by 31.1%, 3.7 °C, and 0.11 °C for the anhydrous ethanol filled MGFHP and 34.4%, 3.1 °C, and 0.13 °C for the hexane filled MGFHP, respectively.

Experimental and numerical study on the thermal and hydraulic characteristics of porous-media heat exchangers in cryogenic conditions

The thermal performance of a metal-foam heat exchanger in cryogenic environments was examined through an experiment and a computational fluid dynamics (CFD) simulation. A plate-type heat exchanger with metal foam inserted in the channels was designed and manufactured. The metal foam was 20-pore-per-inch nickel metal foam and brazed to stainless-steel channel plates. Nitrogen gas in cryogenic conditions was supplied to both hot and cold-side channels with a counter-flow configuration. The pressure drop was measured in a wide range of mass-flow rates of 8–48 kg/h. Heat transfer experiments were conducted while adjusting the gas temperature using heating wires in a range of mass-flow rate of 10–20 kg/h. The data obtained through the experiment were compared with those of CFD simulations and previous correlations obtained at room-temperature conditions. The CFD prediction of the pressure drop agreed well with the experimental data with less than 10 % discrepancy. Furthermore, the pressure-drop correlation models at room-temperature conditions agreed well with the experimental results in cryogenic conditions. Among the pressure-drop models, the Dietrich model showed the best agreement. CFD predicts the heat transfer well within 10 % discrepancy from the experiment. However, the heat transfer in cryogenic conditions is 15–20 % higher than that of the previous correlations obtained at room-temperature conditions. This can be explained by the hydraulic performance not being affected much by the temperature difference, while the Nusselt number of the metal-foam-filled channel has higher value due to lower thermal conductivity at a cryogenic environment.

Experimental assessment of the similarity law for a heat conduction problem

Similarities are mathematical laws, which are based on the dimensionless formulation of the governing equation of a physical phenomenon. Consequently, a set of characteristic dimensionless numbers is obtained. Similarities aim, via these numbers, to find equivalences between particular configurations. This approach is widely adopted, especially in fluid and aerodynamic problems. In the building context, the phenomena of heat and mass transfer in porous media occur with slow kinetics. Thus, similarities can be employed to reduce the duration of experimental campaigns to characterize material properties. These laws were investigated here in the case of a heat transfer problem through an experimental campaign. Two equivalent configurations were submitted to a heat stress. Temperatures inside materials were measured and compared to assess the validity of thermal similarity. A complete evaluation of uncertainty propagation was then carried out. Uncertainties related to the sensor position, its response time, the omission of mass transfer, the sensor systematic accuracy, the random measurement aspect, the one-dimensional transfer hypothesis and the boundary condition modeling were evaluated. The comparison of both configurations was carried out based on the confidence interval of both measurements. The results showed a good agreement between the reference and reduced experiment. On the basis of these findings, similarities were experimentally verified within a margin of discrepancy that was justified. Thus, they can be adopted in heat transfer experiments in order to identify equivalent configurations that are easier to conduct.

New streamlined catalytic carriers of enhanced transport properties: Experiments vs CFD

The paper presents the research on the new structured catalytic carriers, called streamlined structures. The innovative structures are composed of short triangular channels, the walls of which are similar to an airfoil profile. The structures were made using additive manufacturing method – selective laser melting (SLM) - in three different lengths. The study includes flow resistance and heat transfer, both realized using Computational Fluid Dynamics (CFD) modeling and experimentally. Flow resistance was measured by precise micromanometer over a wide range of gas velocities. During heat transfer experiments, electric current flowing through the metal structures heated them (Joule effect). The experimental results are in acceptable agreement with the CFD taking into account the accuracy of the structures' manufactured. The novel structures display improved heat transfer properties compared to monoliths and satisfactory low pressure drop. The flow patterns modelled using CFD prove that the inlet vortex has completely disappeared and the outlet vortex is significantly reduced compared to classic monolithic structures. The novel structure studied here is a promising prospect for the development of catalytic reactors.

Experimental investigation on the flow condensation of R450A, R515B, and R1234ze(E) in a 7.0 mm OD micro-fin tube

The flow condensation of R1234ze(E), R450A, and R515B was conducted in a 6.14 mm inner diameter micro-fin tube. The fin height is 0.18 mm and the fin number is 50. The helix angle is 18° and the area enhancement factor is 1.63. During the experimental tests, the saturation temperature is 30 and 40 °C. The mass fluxes range from 50 to 400 kg m-2 s-1. The vapor qualities are between 0.1 and 0.96. The heat transfer coefficients and frictional pressure gradients of the three refrigerants were compared with each other. The experimental heat transfer coefficient increases with vapor quality and mass velocity, except at high vapor quality, where a higher mass velocity does not imply a higher heat transfer coefficient, probably due to the liquid film thickness distribution. Generally speaking, R450A presents slightly higher heat transfer coefficients at high mass fluxes and smaller pressure gradients, whereas R515B shows lower heat transfer coefficients and pressure drops very close to those of R1234ze(E). Moreover, some models of the heat transfer coefficient and pressure gradient specifically developed for micro-fin tubes were chosen to explore their feasibility for the new refrigerants and tube. Cavallini et al.’s model shows the best agreement for the heat transfer coefficient, and Diani et al.’s model shows the best agreement for the frictional pressure gradient.

In tube condensation heat transfer and pressure drop for R454B and R32—Potential replacements for R410A

The heating, ventilation and air conditioning (HVAC) industry in the United States seeks near-term alternative refrigerants to replace R-410A in unitary equipment. Two potential replacement refrigerants are R-454B and R-32. It is of interest to investigate the capability and accuracy of existing heat exchanger design methods when applied to these replacement refrigerants. To that end, this work presents empirical condensation quasi-local heat transfer coefficient and pressure drop data for R-454B and R-32. These data were obtained in a 38 in. (9.52 mm) outside diameter (OD) smooth copper tube with a wall thickness of 0.032 in. (0.81 mm). The experimental variables and their ranges included refrigerant absolute pressure (1960≤Pabs≤3196 kPa), condensation temperature (35≤Tcond≤50oC), mass flux (100≤G≤200 kg m-2 s-1), vapor quality (0≤x≤1), and heat flux (32.9≤q″≤62.97 kW m-2). It was found that the heat transfer correlation developed by Cavallini et al. Cavallini et al. (2006) predicted the experimental condensation heat transfer data, for both R-454B and R-32, with the greatest accuracy. Using the Cavallini et al. correlation, it was found that the mean absolute percentage error (MAPE) was 10.5% and 15.1% for R-454B and R-32, respectively. Additionally, the pressure drop correlation developed by Friedel Friedel (1979) predicted the experimentally determined pressure gradient with a MAPE of 7.7% and 5.5% for R-454B and R-32, respectively. These results will assist the practicing thermal engineer to choose the most appropriate design correlation for these near-term replacement refrigerants.

Heat Flux on a Hypersonic Cone with a Highly Swept Fin

A detailed comparison between computations and experiments is given for Mach 6 flow over a straight cone with a highly swept fin. Five cases are simulated that vary the cone nose radius, fin thickness, fin sweep angle, and freestream unit Reynolds number. Experimental data are available for four cases. Surface heat flux comparisons between steady, laminar simulations and experimental measurements are made on the surface of the cone and fin. Uncertainty for both computations and experiments is estimated. Three cases are identified as transitional or turbulent, while one case is identified as laminar. Estimates of transition location are derived from experimental heat transfer and measurements of surface pressure fluctuations over a range of unit Reynolds numbers. The effect of geometric parameters is deduced. Further investigation into geometric changes demonstrates the sensitivity of the finned cone flowfield (e.g., vortex location and strength) to the shape of the fin–cone junction. Additionally, simulations demonstrate that the flow on the fin and cone is largely unaffected by truncating the fin. Changes in the flowfield remain localized to the fin truncation.

Effect of Surface Topology on the Apparent Thermal Diffusivity of Thin Samples at LFA Measurements.

This paper deals with the problem of the influence of surface topography on the results of thermal diffusivity measurements when determined using the instantaneous surface heat source method, also called the pulse method. The analysis was based on numerical tests carried out using Comsol Multiphysics software. The results of experimental investigations on the actual material structure using an electron microscope, an optical microscope and a profilometer were used to develop a numerical model. The influence of the non-uniformity of the surface of the tested sample on the determined values of half-time of the thermal response of the sample’s rough surface to the impulse forcing on the opposing flat surface was determined by developing the data for simulated measurements. The effect of the position of the response data reading area on the obtained simulation results was also analyzed. The obtained results can be used to improve the accuracy of experimental heat transfer studies performed on thin-film engineering structures depending on the uniformity and parallelism of the material applied to engineering structures. The difference in half-life determination error results for various analyzed models can be as high as 16.7%, depending on the surface from which the responses of the heating impulse are read. With an equivalent model in which 10% of the material volume corresponds to the rough part as a single inclusion, hemisphere, the error in determining thermal diffusivity was equal to 3.8%. An increase in the number of inclusions with smaller weight reduces an error in the determination of thermal diffusivity, as presented in the paper.

Thermal characterization of spray impingement heat transfer over a High-Power LED module

• Thermal management of single nozzle spray for a high-power LED module is explored. • Maximum experimental heat transfer coefficient ∼12 kW/m 2 K is obtained for Re of 12000. • LED substrate temperature is maintained well below safe limits for Re = 8000 and above. • Spatial and temporal thermal fields over the LED module is computationally estimated. • The experimental results are complemented with 3D numerical heat transfer simulations. The cooling demand for a nominal 300 W power LEDs is around 200 W/cm 2 at the chip-scale, and the junction temperature must be maintained below 120 °C for reliable operation. Special thermal management packaging is required to maintain LEDs below this reliability temperature limit. The present study investigates the thermal characteristics of single-nozzle spray cooling over a high-power LED module. The detailed thermal characterization within the LED assembly is explored using both, experimental and numerical approaches. The LED substrate temperature is experimentally obtained for various input power supplies, water flow rate, inlet water temperature, nozzle height, and offset from the LED center. Heat transfer coefficient at two radial locations ( R = 0 mm and 12.5 mm) is estimated to evaluate the heat removal capacity of the spray for these operating conditions. Numerical study is performed to visualize temperature and heat flux distribution within the LED module, and to investigate the appearance of thermally critical locations. Junction temperature is the critical parameter for thermal characterization of the LED module, and is numerically investigated for various operating conditions. The junction temperature is maintained below 95 °C at the nominal electrical input power for a Re ≥ 8,000 using the proposed spray cooling design. The present study establishes the efficacy of spray cooling for high-power LED modules, even when the supply power exceeds 112% of the nominal power range.

Pembuatan model alat pembuang panas untuk air pendingin alat praktikum heat exchanger merk PA Hilton seri H101 di laboratorium teknik mesin

Heat Exhanger Service Unit H101/08490 P.A Hilton Ltd that use for heat transfer experiment has a weakness because the water flow from the unit after taking heat will directly enter the storage tank, still have a high temperature ( >33 o C ). The use of practicum equipment for a experiment with a long enough duration will cause the cooling water are used have a high temperature and be ineffective for the heat exchanger cooling process. The purpose of making a Cooling Tower Type Heat Dissipator Model for Heat Exhanger Service Unit H101/08490 P.A Hilton Ltd so that the cooling water used to carry out the practicum will be maintained at a temperature of 28 o C-33 o C. Maintaining the stability of the cooling water temperature is expected to produce data that does not vary too much and can be done in a fairly long data collection range. The results of the performance and effectiveness test of cooling tower cooling show that the temperature increase is only 4 o C for 3 hours of testing and the cooling effectiveness increases and is relatively constant during the test time. This shows that the cooling system with a cooling tower can be used as a supporting medium for heat exchanger practicum in the Mechanical Engineering department.

Influence Analysis of Convective Heat Transfer of Circular Tubes and Flat Plates in a Polar Environment

Electric heating serves as a major cold-proof measure because engineering equipment often loses efficacy in harsh weather conditions in a polar environment. Because Convective heat transfer is the key issue to solve the cold-proof design, this paper analyzes convective heat transfer of tube and plate in the constant heat flow heating mode. According to fluent numerical simulation analysis with a wind speed of 0–40 m/s and a temperature of −40°C–0°C, both an increase in wind speed and a decrease in temperature would cause the growth of the convective heat transfer coefficient of circular tubes and flat plates, Wind speed displayed a more significant impact on convective heat transfer. In a low-temperature laboratory, a model experiment of convective heat transfer was carried out to validate the reliability of the numerical simulation. Last, a prediction model for convective heat transfer coefficient of circular tubes and flat plates in polar environment was developed using numerical and experimental data.

Thermal performance of microencapsulated phase change material slurry in helical coils with reversed loops and wire coil inserts

ABSTRACT In this study, the flow and heat transfer characteristics of microencapsulated phase change material (MPCM) slurry were experimentally investigated using a newly designed helical coil heat transfer device. The conventional helical coil has been structurally modified with passive enhancement features aiming to further promote fluid mixing. Specifically, 360° plastic tubing with or without wire coil inserts was added after each 180° of the main helical loop to enhance fluid mixing and improve the overall thermal performance of the device. Pressure drop and heat transfer experiments with MPCM slurry were conducted under turbulent flow and constant heat flux conditions. A new friction factor and Nusselt number correlations for MPCM slurry in helical coils with reversed loops and wire coil inserts are proposed. Experimental results show that the structural modifications did enhance the heat transfer performance of MPCM slurry. The experimental results revealed that the phase change process of MPCM considerably enhanced the heat transfer rate of MPCM slurry. Furthermore, the use of reversed loops and wire coil inserts led to better fluid mixing within the coil, resulting in improved convective heat transfer of the MPCM slurry.

The investigation of heat transfer enhancement by using different mixture conditions of graphene nanofluids on a downward facing surface

Abstract In this study, graphene nanofluids were used to explore the effect of various concentrations on boiling heat transfer of downward-facing heating. Five concentrations of graphene nanofluids were prepared for pool boiling heat transfer experiments. The experimental results show that when the mass concentration is 10 mg/L, the maximum enhancement of the CHF is up to 76.1%. In order to explore the mechanism of graphene nanofluid enhancing boiling heat transfer, after the experiment, the wettability and roughness of the heating surface were measured and the heating surface was characterized by a scanning electron microscope (SEM) and electronic differential system (EDS). The results show that the wettability is enhanced and the surface roughness is reduced. In addition, boiling curves (the curves of heat flux with surface superheat) and the curves of heat transfer coefficient with heat flux at different concentrations have also been observed to further explore the mechanism of enhanced heat transfer.

Development and applicability of heat transfer analytical model for coaxial-type deep-buried pipes

Based on the infinite line source model and logarithmic mean temperature difference, an analytical model for solving the heat transfer of coaxial-type deep-buried pipe has been proposed in this investigation. The actual assignment of vertical lithology and temperature values of the ground around the buried pipes could be realized by layered modeling. To validate the analytical model in terms of reliably calculating the real-time and on-way water temperature of buried pipes, the field experiment of buried-pipe heat transfer (at a depth of 2539 m in Xi'an) was carried out, and the numerical model developed based on the experiment was employed for corroboration. Further, by setting combined conditions of multiple factors affecting the heat transfer of buried pipes, the applicability of the analytical model under different model parameter combinations was substantiated, and the main factors affecting the calculation accuracy of the analytical model were analyzed. The results showed that the relative error between the analytical solution and experimental value was less than 5.92% for the buried-pipe heat transfer intensity under the experimental condition. The relative error between the analytical and numerical solutions was less than 6.20% under the combined conditions of multiple factors.

The performance evaluation for thermal protection of turbine vane with film cooling and thermal barrier coating

• An expression of temperature-independent overall cooling effectiveness is obtained. • The external convective heat transfer is effected by the internal cooling flow. • Increasing convective heat transfer can increase overall cooling effectiveness. • Four parameters need to be similar when conducting heat transfer experiments. When conducting wind tunnel heat transfer test to evaluate the cooling performance of the turbine, it is difficult and costly to simulate the extremely high temperature gas of the real working conditions, and small temperature difference and room temperature test is usually used instead. In order to evaluate the cooling performance under real working conditions from the experimental results at room temperature, the corresponding evaluation method needs to be studied. In this paper, a turbine vane with film cooling and thermal barrier coating (TBC) is used as the study object. By introducing an improved driving temperature and adopting one-dimensional heat transfer analysis, an improved analytical expression of overall cooling effectiveness, which is independent of temperature, is derived and analyzed. Through numerical calculation, the overall cooling effectiveness defined by different reference temperatures are compared. The results show that the heat transfer of the turbine vane with film cooling and TBC is complicated. The influence of the internal cooling should be considered when calculating the external convective heat transfer. The temperature-independent overall cooling effectiveness on the TBC surface and the superalloy surface are obtained, respectively. The improved overall cooling effectiveness is more sensitive to the thermal resistance of the superalloy compared with the thermal resistance of the TBC. The overall cooling effectiveness can be improved by increasing the internal or external convective heat transfer coefficient. In order to obtain an accurate overall cooling effectiveness (dimensionless wall temperatures), it is necessary to satisfy similarities of four dimensionless parameters, including the Biot numbers ( B i TBC , B i w ), the ratio of external and internal convective heat transfer coefficients ( h e / h i ) and the ratio of gained convective heat transfer coefficient and external convective heat transfer coefficient ( h e , i / h e ).

Data-Driven Calibration of Rough Heat Transfer Prediction Using Bayesian Inversion and Genetic Algorithm

The prediction of heat transfers in Reynolds-Averaged Navier–Stokes (RANS) simulations requires corrections for rough surfaces. The turbulence models are adapted to cope with surface roughness impacting the near-wall behaviour compared to a smooth surface. These adjustments in the models correctly predict the skin friction but create a tendency to overpredict the heat transfers compared to experiments. These overpredictions require the use of an additional thermal correction model to lower the heat transfers. Finding the correct numerical parameters to best fit the experimental results is non-trivial, since roughness patterns are often irregular. The objective of this paper is to develop a methodology to calibrate the roughness parameters for a thermal correction model for a rough curved channel test case. First, the design of the experiments allows the generation of metamodels for the prediction of the heat transfer coefficients. The polynomial chaos expansion approach is used to create the metamodels. The metamodels are then successively used with a Bayesian inversion and a genetic algorithm method to estimate the best set of roughness parameters to fit the available experimental results. Both calibrations are compared to assess their strengths and weaknesses. Starting with unknown roughness parameters, this methodology allows calibrating them and obtaining between 4.7% and 10% of average discrepancy between the calibrated RANS heat transfer prediction and the experimental results. The methodology is promising, showing the ability to finely select the roughness parameters to input in the numerical model to fit the experimental heat transfer, without an a priori knowledge of the actual roughness pattern.

Optimisation of wasted air utilisation in thermal loss reduction in double-glazed windows of commercial buildings in cold regions

Ventilating of multi pane-glazed windows using wasted air of buildings is an effective technique to minimize heat loss through windows and save heating energy in cold regions. In low-scaled occupancy buildings with high WWR ratio, buildings supply a low flow rate of wasted air to windows ventilation systems, resulting in a declination in its thermal performance. Therefore, this study introduces methods of managing the utilisation of wasted air in windows ventilation to optimise the energy saving. Two methods have been implemented experimentally on a small-scaled room. The first method is a time-based division of air pump operation, an air pump ventilates multiple windows, one window at a time repetitively. The second method shares the available wasted air to multiple windows. The experimental results and mathematical heat transfer model have been employed to evaluate thermal performance of the system in different methods. The first method showed a best energy saving with a duty cycle of 50% for the air pump, and on/off operation every 10 s. An energy saving of 42.6% has been realized compared to the traditional double-glazed windows, and the heat transfer coefficient was declined from 3.82 to 2.8 W/m2 K. The second method showed an optimum thermal performance when the available flow rate of wasted air was shared with three double-glazed windows. An energy saving of 83.1% was achieved compared to the traditional double-glazed windows, and the heat transfer coefficient dropped from 3.82 to 2.36 W/m2 K.

Research on heat transfer mechanism and performance of a novel adaptive enclosure structure based on micro-channel heat pipe

In this paper, a novel adaptive enclosure structure (AES) combining the building wall and heat pipe is proposed for building wall energy saving. The theoretical heat transfer process of the structure is analyzed, and its heat transfer performance is obtained. The theoretical thermal resistance of the structure is about 0.165 K/W. The one-dimensional steady-state heat transfer experiment has been carried out. Under the temperature difference of 35 °C, the surface temperature of the AES is much higher than that of the traditional wall on the heating demand side (about 9.5 °C). The heat transfer experiment under the influence of multiple factors has been carried out. Under heating demand, the indoor side surface temperature of the AES is higher than that of the traditional wall (about 1.5 °C). Under cooling demand, the indoor side surface temperature of the structure is lower than that of the traditional wall (about 0.8 °C). In addition, it is realizable to switch the performance of the AES between efficient heat transfer or good thermal insulation according to requirements. The proposed structure has great potential for reducing the indoor energy consumption and improving thermal comfort, and could provide a new solution for passive energy-saving technology based on building envelope.

Flow and heat transfer characteristics of coal-based rocket kerosene in mini-tube with ultra-high parameters

Coal-based rocket kerosene as a new aerospace fuel and active regenerative cooling technology coolant, in order to make it widely used in engineering, the flow and heat transfer experiments were carried out for the first time under the actual working parameters of the rocket engine. The effects of heat flux (2–33.8 MW·m−2), mass flow rate (8000–48,000 kg·m−2·s−1), and inlet temperature (323–473 K) were investigated. The results indicated that as the heat flux increases, the total pressure drop and friction factor first decreased and then increased. With Reynolds numbers 3.1 × 104 and 7.6 × 104 as the boundary, the flow was in the hydraulic smooth tube region, transition region and turbulence rough tube region respectively. Increasing the inlet temperature can inhibit the heat transfer deterioration caused by the entrance effect. Higher heat flux can be maintained by increasing flow rate. When the wall temperature just exceeded the pseudo-critical temperature, the rate at which the heat transfer coefficient increased with the increase of heat flux became higher. Considering the influence of various parameters to modify classic correlation, and using the probability density function to modify the thermophysical properties of coal-based rocket kerosene, a new Nusselt number correlation was proposed.

Home Experiment Program for Senior Mechanical Laboratory Course: A Laboratory Program during COVID-19 Pandemic

COVID-19 pandemic triggers the development of virtual methods in doing the experiments for laboratory classes. However, the virtual methods cannot give the students a hands-on learning experience they should get from laboratory classes. This paper aims to disseminate the Home Experiment program conducted by Mechanical Engineering study program of Sampoerna University. This program is established to allow students to conduct experiments at home so that they obtain the hands-on learning experience during the pandemic. The program is conducted through the Senior Mechanical Laboratory (SML) course, and the result from heat transfer experiment is discussed in this paper. Students are assigned to design the heat transfer experiment (guided by the lecturer), purchase the required components, assemble the components, and conduct the experiment. Aside from submitting a written report, students are also required to submit experimentation videos and present their work in a conference at the end of the semester. It is hoped that this program will become a study case for home-based learning which is crucial during the pandemic situation.