Applied Heat Transfer

It is extremely important to gather the viscosity behavior of fluids accurately for industries and academia. There is no better method than viscosity measurement to detect changes in the specific characteristics of the materials. However, viscosity measurement is indeed a very sensitive process. In nature, fluids are involved in widely various containers, and they are affected by serious temperature deviations. It is a necessity for viscometers to have the capability to obtain accurate data from all types of containers of fluids even in serious temperature variations in order to understand the natural phenomena inside the fluids. Conventional viscometers mainly neglect the effect of sudden temperature deviations inside the fluids, or they need to use very expensive water bath systems that stabilize the temperature around the test fluids, which is not feasible at all. In this research, the effects of non-uniform temperature fields are analysed detailly to confirm that even in extremely limited amounts, serious viscosity and temperature deviations may occur. Experiments were performed in two parts. The first part was conducted using several thermocouples with different test fluids to find out the effects of thermal conduction and convection. For the second part, particle image velocimetry (PIV) was utilized to comprehend the flow movement within the test fluids. It was shown that even for small volumes and even in very controlled environments, an almost 35% viscosity measurement error (VME) occurs. Finally, as a solution to this problem, a new non-dimensional parameter called the Akpek number was proposed. The Akpek number enables the estimation of VMEs in any possible case. VME is a very crucial obstacle that has urgency to be illuminated by researchers and scientists to improve fluid characteristics. The main goal of this research is to illuminate the importance of this problem and offer a potential solution. The final results are supported by experimental data and numerical simulations using OpenFOAM. Full article

With the rapid progress in data mining, deep learning, and artificial intelligence, the demand for datacenters of various sizes increases globally. Datacenters typically require an environment with properly controlled temperature and humidity conditions for their proper operations. These needed environmental conditions are always provided by an air conditioning system. In humid and hot regions, both energy consumption and the splash of water condensate in using the fin-and-tube heat exchangers are of concern because reliability issues can occur. In this study, the effects of fin surface hydrophilic/hydrophobic coatings on the performance of the fin-and-tube heat exchangers, including the heat transfer rate, pressure drop, and water-condensate splash, were investigated experimentally. By varying the cooling air speeds and fin pitches, the results show that hydrophilic surface coating is an effective method in reducing both the pressure drop (thus saving energy) and the condensate splash, while not affecting the heat transfer rates significantly. The water splash reduction is achieved by both the increased air speed for splashing and a smaller amount of splashing. Water splash can even be completely eliminated if the airspeed was below about 3 m/s. In contrast, hydrophobic surface coating will increase both pressure drop and water splash; thus, should be applied with caution. Full article

Knowledge of thermophysical properties of materials is important in the design process to meet the ambitious targets with respect to reliability and performance of many modern machinery. In this paper a simple method for the measurements of thermophysical material properties is presented. A bar of the sample material is heated at one end by a constant heat source and temperature sensors on or in the sample material at different locations record the temperature response. In the limit of small Fourier-Numbers the temperature will not rise at the adiabatic end and the comparison to the theoretical curve allows to extract thermophysical data. In the case of large Fourier-Numbers a quasi steady temperature profile in the bar allows to extract all relevant thermophysical properties simultaneously. Apart from the theory some measurement results are presented and the errors due to diabatic boundary conditions are discussed. Full article

The classic equations used to find the form factor inside fragments of spheres are often unassailable. The main difficulties that they present lie in iterative integrations effected over curved surfaces. The typical simulation software for this kind of issue is not capable of tackling the drawbacks that appear in the process, among them we could cite the impossibility of discretizing curved shapes with equal matching tiles, whether triangles or rectangles, especially when we arrive at the contour elements. The current type of cylindrical tiles employed for the calculation of spheres, due to incoherence in curvature, presents a significant array of gaps that render the whole procedure inadequate and inconsistent. To countermeasure this drawback, the recent finding of some innovative principles by the present author has provided a sure and exact path towards the solution of the problem in the frequent case of a volume enclosed within a spherical fragment and two limiting sections of the said sphere placed at arbitrary positions. The coherent application of such postulates by virtue of form factor algebra leads to an encompassing expression which solely requires the input of the surface areas of the involved shapes and, thus, avoids the lengthy resort to integration. A relevant number of cases in radiative heat transfer simulation, that cannot be solved by any other method, become feasible and accurate. Since the new tool can be implemented as an algorithm for simulation software, pivotal advances emerge in the complex domain of radiation which are applicable for the lighting industry, building simulations, and aerospace technologies, among others. Full article

The thermal comfort and air-conditioning energy consumption of vehicles or trains are dependent on the thermal resistance of its envelopes, which could be enhanced by improving the radiation characteristics of the narrow cavities scattered in their envelopes. However, the study for a feasible method and its effectiveness has been given little attention. This paper introduces a method that involves pasting aluminum foil on the inner wall to change the radiation characteristics of the narrow cavity and analyzed its effects on narrow-cavity heat transfer by experimental and numerical methods. The results indicate that the radiation effect on heat transfer in a narrow cavity made of conventional material is dominant, with a rate larger than 75%, and that pasting aluminum foil is an effective and feasible method of weakening the radiation rate in narrow-cavity heat transfer, decreasing it to less than 10%. This paper will provide a reference to improve the insulation characteristics of vehicle enclosure. Full article

An experimental study on natural convection heat transfer from the outer surface of a horizontal array of vertical square tubes in the air is investigated. The array consists of three vertical square tubes at equally different center-to-center distances. Each tube has a square cross-section with a side length of 2.00 cm, 100 cm length, and is filled with sand. Each tube is heated by inserting an internal heating element with a constant heat flux at the center. Five center-to-center separation distance to hydraulic diameter ratios (S/D) are used at different heat flux ranges of 70–360 W/m². Results show that at small S/D, the Nusselt number of any tube in the array is lower than that of the single tube up to a specific S/D and then increases as the ratio increases. Empirical correlations are obtained for each tube in the array at different S/D using the modified Rayleigh numbers only. General correlations using S/D as a parameter are obtained for each tube, and an overall general correlation using both S/D and the tube number (n) as parameters is obtained. The difference between the predicted and experimental Nusselt numbers is in the reasonable range even at high Rayleigh numbers. Full article

Metal hydride (MH) hydrogen storage needs an external heat source to release the stored hydrogen. To enhance the thermal performance of MHs, the incorporation of phase change materials (PCM) is a way to preserve reaction heat. This work proposes a new MH-PCM compact disk configuration (i.e., a truncated conical MH bed surrounded by a PCM ring). An optimization method is developed to find the optimal geometrical parameters of the MH truncated cone, which is then compared to a basic configuration (i.e., a cylindrical MH surrounded by a PCM ring). Moreover, a mathematical model is developed and used to optimize the heat transfer in a stack of MH-PCM disks. The optimum geometric parameters found (bottom radius of 0.2, top radius of 0.75 and tilt angle of 58.24) allow the truncated conical MH bed to reach a faster heat transfer rate and a large surface area of higher heat exchange. Compared to a cylindrical configuration, the optimized truncated cone shape enhances the heat transfer rate and the reaction rate in the MH bed by 37.68%. Full article

The trend of miniaturisation in recent decades has led to the development of compact electronic devices. The reduction in the required dimension leads to the exponential rise in the heat flux dissipated from such a system. A proper thermal management system is necessary to keep the temperature of a computer chip’s junction within acceptable limits and maintain its performance. Flow boiling modification using straight fins in microchannels has proven to be an effective passive enhancement of the cooling system. The core interest of this research is figuring out the optimal configuration of the fin shapes and configurations. Hence, it is crucial to gain a comprehensive understanding of the flow boiling phenomenon to establish a more general approach. In this study, the boiling heat transfer performance of fin microchannels with various shapes and dimensions is investigated experimentally. The study has shown that the choice of fin geometry has a significant impact on the thermal performance of a heat transfer system. Specifically, the results indicate that a rectangular cross-section fin performs better than a trapezoidal one with the same fin gap. The rectangular cross-section fin exhibits the highest heat transfer coefficient of 5066.84 W/m²∙K, outperforming the trapezoidal fin in terms of heat transfer capability. As the hydraulic diameter reduces, the thermal boundary layer becomes denser, providing a more distributed saturated region. This leads to the increase in the heat transfer coefficient up to 22.5% and 17.1% for rectangular and trapezoidal fins, respectively. Additionally, the efficiency analysis shows that, albeit increasing the mass flux and reducing the gap increase the average cooling performance, but the pressure drop jumps up to 48%, reducing the efficiency of the heat removal system. Full article

The heating of oil and oil products is widely used to reduce energy losses during transportation. An approach is developed to determine the effective length of the heat exchanger and the temperature of the cold coolant (oil) at its outlet in the case of a strong dependence of oil viscosity on temperature. Oil from the Uzen field (Kazakhstan) is considered as a heated coolant, and water is considered as a heating component. The method of the log–mean temperature difference, modified for the case of variable viscosity, and the methods of computational fluid dynamics (CFD) are used for calculations. The results of the numerical calculations are compared with the data obtained on the basis of a theoretical approach at a constant viscosity. When using a theoretical approach with a constant or variable viscosity, the heat transfer coefficients to cold and hot coolants are found using criterion dependencies. The Reynolds-averaged Navier–Stokes (RANS) and a turbulence model that takes into account the laminar–turbulent transition are applied. In the case of variable oil viscosity, a transition from the laminar flow regime to the turbulent one is manifested, which has a significant effect on the effective length of the heat exchanger. The obtained results of the CFD calculations are of interest for the design of heat exchangers of a new type, for example, helicoid ones. Full article

This study investigates the use of transfer learning and modular design for adapting a pretrained model to optimize energy efficiency and heat reuse in edge data centers while meeting local conditions, such as alternative heat management and hardware configurations. A Physics-Informed Data-Driven Recurrent Neural Network (PIDD RNN) is trained on a small scale-model experiment of a six-server data center to control cooling fans and maintain the exhaust chamber temperature within safe limits. The model features a hierarchical regularizing structure that reduces the degrees of freedom by connecting parameters for related modules in the system. With a RMSE value of 1.69, the PIDD RNN outperforms both a conventional RNN (RMSE: 3.18), and a State Space Model (RMSE: 2.66). We investigate how this design facilitates transfer learning when the model is fine-tuned over a few epochs to small dataset from a second set-up with a server located in a wind tunnel. The transferred model outperforms a model trained from scratch over hundreds of epochs. Full article

The selection of a battery thermal management technique is important to overcoming safety and performance problems by maintaining the temperature of batteries within a desired range. In this study, a LiFePO₄ (LFP) pouch-type battery having a capacity of 20 Ah was experimentally cooled with both air and liquid (immersion cooling) techniques. Distilled water was selected as the immersion fluid in the experiments, and the impact of discharge rate (1–4C), immersion ratio (50–100%), and coolant fluid inlet temperature (15–25 °C) on the battery temperature were investigated during the discharge period. The experiments revealed that maximum temperatures were reached at approximately 45 °C and 33 °C for air and distilled water cooling techniques, respectively, at the discharge rate of 4C. The average and maximum battery surface temperatures can be reduced by 28% and 25%, respectively, with the implementation of the liquid immersion technique at the discharge rate of 4C compared to the air technique. Moreover, the experiments demonstrated that the maximum temperature difference could be lowered to 4 °C by means of 100% liquid immersion cooling at the highest discharge rate, where they are approximately 11 °C and 12 °C for air and 50% for immersion cooling, respectively. In addition, it was observed that the coolant fluid inlet temperature has a significant impact on battery temperature for %100 liquid immersion. Full article

This work deals with the application of the temperature oscillation method to measure local values of the heat transfer coefficient in laminar flow in a straight horizontal pipe and in a pipe with a bend. This method, applied here for the first time in such experimental conditions, uses a time-varying heat flux as a boundary condition, which is a condition of the third kind. Since an analytical solution for such conditions could not be identified in the existing literature, a numerical study of this problem is applied here. Experimental data for a straight horizontal pipe confirm the numerical results within a reasonable level of error. Experimental measurements on a straight horizontal tube are consistent with predicted results in the literature for both total and local heat transfer coefficient values. The measurements on bent tubes show a very significant influence on the local values compared to the straight tube and in the overall values this change appears as an increase in the heat transfer coefficient. Full article

The gas turbine blades are scoured by high temperature gas sustainedly and long-term in harsh environment. It is of great significance to explore effective cooling methods to lower the turbine blade temperature so as to ensure safe and stable operation of the gas turbine. However, there are few studies on the cooling channel considering the turning vane, variable cross-section characteristics, and rotation effect. In this paper, five kinds of serpentine cooling channel models with variable cross-section properties and different thickness guide vanes are constructed. The effects of different thickness guide vanes on the overall performance of the channel under stationary and rotating conditions are discussed and compared by numerical method. The result shows that when stationary (Re = 10,000–50,000), the turning vane with suitable thickness can increase the Nu/Nu₀ by 56.5%. The f/f₀ is decreased by 14.2%, and the comprehensive thermal performance is increased by 4.5%. When rotating (Re = 10,000, Ro = 0–0.5), the turning vane with suitable thickness can increase the Nu_up/Nu₀ and Nu_all/Nu₀ by 33.0% and 4.0%, respectively. The comprehensive performance of the variable cross-section serpentine channel can be greatly improved by arranging the turning vane structure with appropriate thickness. Full article

A numerical simulation on the heat transport augmentation and flow drag behavior of spirally corrugated pipes was performed. The simulation was conducted on the basis of the experimental results documented in the published literature. The influence of the thread height and pitch on the hydraulic–thermal performance as well as the mechanism of the convection heat transport development inside the spirally corrugated pipe were explored. It was discovered that the convection heat transport performance elevates in the Reynolds number region of 4000~13,000 as the thread height rises or the Reynolds number enlarges, but it declines when the thread pitch extends. The convection heat transport performance marked by the Nusselt number of the spirally corrugated pipe could reach 2.77 times that of the plain pipe, while the flow resistance coefficients of spirally corrugated pipes are 89~324% above that of the plain pipe. It enlarges with the rise in thread height but declines with the extension of the thread pitch. It also reduces when the Reynolds number enlarges. The factors of overall heat transmission performance for all the spirally corrugated pipes are above 1.00, and they increase in the Reynolds number region of 4000~7000 and then decrease in the Reynolds number region of 7000 to 13,000. The secondary flow at the cross-sections and the vortex between two adjacent corrugated grooves are the basic causes of the promotion of convection heat transport inside the spirally corrugated pipes. The secondary flow near the pipe wall both disrupts the border layer and boosts the radial interfusion of the fluid. In addition, the existence of vortexes makes the secondary flow act on the convection heat transmission continuously and positively in the region close to the pipe wall. Full article

Thermal management at a high heat flux is crucial for high-power electronic devices, and jet impingement cooling is a promising solution. In this paper, a hybrid heat sink combining a microchannel and jet impingement was designed, fabricated and tested in a closed-loop system with R134a as the working fluid. The thermal contact resistance was measured by using the steady-state method, and the thermal resistance of the heat sink was obtained at different heat fluxes and flow rates. The maximum heat dissipation of 400 W/cm² is achieved on a heater area of 210 mm², and the thermal resistance of the heat sink is 0.11 K/W with a pressure drop of 13.5 kPa under a flow rate of 1.90 L/min. Low thermal resistance can be achieved for the hybrid heat sink stemming from the highly-dense micro-jet array with separate inflow and outflow microchannels. Full article