Heat Transfer Thermal Resistance Research Articles

Characteristics analysis and structure optimization of a hybrid micro-jet impingement/micro-channel heat sink

To solve the heat dissipation problem of high-power electronics, a novel hybrid micro-jet impingement/micro-channel heat sink is proposed in this work. Different from the continuous and straight rectangular micro-channel, the inclined and interrupted channel walls (fins) are adopted to enhance the fluid disturbance inside the heat sink. To obtain the thermal–hydraulic performance of the hybrid heat sink, an overall 3D modeling and numerical simulations are conducted instead of local cell numerical simulation. In addition, to further improve the heat transfer and flow resistance performance, the effects of variation of structural parameters on heat transfer and flow resistance properties are investigated, including the fin inclination angle, fin length and fin height. The results of structural parametric analysis reveal that there are interactions between different structural parameters, but with different levels of significance for different evaluation indicators. Then, to obtain the optimal combinations of structural parameters for different applications, the multi-objective genetic algorithm coupled with artificial neural network (ANN) is utilized to search for the optimal value of each structural parameter. The heat transfer thermal resistance, pressure drop and mean absolute temperature deviation of the heated surface are utilized as the optimization objectives to evaluate the cooling and resistance characteristics. The optimization results obtained by three-objective optimization and two-objective optimization are analyzed and compared. Finally, seven optimal compromise solutions are selected from the Pareto front by using TOPSIS algorithm with different weight factor matrices. For the optimal compromise solutions, the maximum relative errors between the predicted values of the ANN and numerical simulation values are only 3%, which further demonstrates the accuracy of the artificial neural network model and the effectiveness of using multi-objective optimization to guide heat sink design.

Heat transfer performance study of a separated pulsating heat pipe

In order to study the heat transfer performance of the pulsating heat pipe (PHP) under the separation arrangement of evaporating and condensing ends, this paper designs a separated PHP for experimental investigation, and describes its heat transfer capability in terms of the wall temperature oscillation performance, the heat transfer temperature difference and the heat transfer thermal resistance. Under different heating powers, the effects of different placement inclinations and their structures on the heat transfer performance of the in-line and bent separated PHP are analyzed. The study shows that in the low heat transfer power scenario, the performance of the in-line type separated PHP is more excellent, and with the increase of the placement of the inclination of the heat transfer situation is further optimized, 90 ° placement of the inclination of the optimal performance of the heat transfer; bent type separated PHP condensing end of a certain level of inclination is conducive to low-power startup and operation, the condensing section of the 15 ° horizontal angle of the optimal performance; and in the high heat transfer power, the two performances are similar, but the arrangement of flexible bent type separated PHP is more suitable for the changing heat transfer scenarios.

Grafted Alkene Chains: Triggers for Defeating Contact Thermal Resistance in Composite Elastomers.

The pursuit of enhancing the heat transfer performance of composite elastomers as the thermal interface materials (TIMs) is a compelling and timely endeavor, given the formidable challenges posed by interfacial thermal transport in the domains of energy science, electronic technology, etc. Despite the efficacy of phase change materials (PCMs) in enhancing composite elastomers' interfacial compatibility, thereby reducing contact thermal resistance for heat transfer improvement, their leakage post-transition has impeded the widespread adoption of this approach. Herein, a strategy is proposed for developing a solid-solid phase change composite elastomer by grafting alkene chains onto the crosslink network to eliminate the possibility of leakage. A series characterization suggest that the resulting material possesses a self-adjusting interfacial compatibility feature to help reduce contact thermal resistance for heat transfer facilitating. The investigations on adhesion strength and surface energy reveal that the presence of amorphous grafted alkane chains at the interface facilitates easier absorption onto the contacting solid surface, enhancing intermolecular interactions at the interface to promote across-boundary heat transfer. By integrating these findings with the thermal performance evaluation of composite elastomers using a real test vehicle, valuable insights are gained for the design of composite elastomers, establishing their suitability as TIMs in relevant fields.

Ensuring the energy efficiency of heat supply energy systems functioning by justifying the choice of glazing units for the external enclosing structures of buildings

The article is devoted to the further development and implementation of technologies that allow some considerable reducing energy consumption and, as a result, reduce greenhouse gas emissions. In Ukraine and abroad, a huge amount of heat is lost through the external enclosing structures of buildings. The largest specific heat losses occur through translucent enclosures, in particular, windows, the main elements of which are double-glazed windows. Today, manufacturers offer double-glazed windows, which are characterized by different costs and energy efficiency. The buyer does not know which double-glazed window to choose. We have proposed a clear and understandable method of solving this issue. For example, two different double-glazed windows were considered and a comparison was made on their cost, thermal resistance of heat transfer, and the ability to save certain amounts of heat and money for its purchase during operation. A feasibility study of additional capital investments in double-glazed windows with improved characteristics instead of conventional double-glazed ones has been carried out. Its results show that, at the current stage of the society development, the use of double-glazed windows with improved characteristics, despite their higher cost, is economically more appropriate than conventional double-glazed windows. This applies to external translucent fences of buildings of any purpose and form of ownership - both residential and industrial, public, which are on the balance sheets of local and state budgets, etc. The results of all calculations are explained in detail and presented in the form of a table. In addition, a simple and clear scheme of technical and economic substantiation of additional capital investments in a double-glazed window with improved characteristics is presented instead of an ordinary one, which can be used at the design stages of new construction or thermal renovation of existing buildings to ensure the selection of the most rational structural solutions. The materials of the article can be generalized to any other cases of technical and economic substantiation of the choice of options for technical solutions in energy-efficient projects.

Experimental study on the structure of the velocity boundary layer in a 3 × 3 rod bundle channel

The velocity boundary layer exists near the fuel rod surface in the fuel assembly due to the viscous effects. The boundary layer forms the main thermal resistance for heat transfer between the rod and the coolant since the momentum and energy transport is weak. In order to investigate the flow characteristics between the rod and the coolant in the rod bundle channel, this study directly measures velocity distribution in the boundary layer using the Particle Image Velocimetry (PIV) technique. The wall friction velocity is obtained by fitting the experimental data to the Spalding formula, choosing 0.436 as the Von Kármán constant, then the dimensionless velocity distribution of the boundary layer is obtained. The dimensionless velocity distribution under different Reynolds numbers is quantitatively studied. In addition, the effect of the gap size on the structure of the velocity boundary layer is investigated. The experimental results indicate that with the increase of the Reynolds number, the range of the inner layer of the boundary layer decreases gradually. In the inner layer of the boundary layer, with the increase of the Reynolds number, the proportion of the viscous bottom layer in comparison to the inner layer decreases, and the proportion of the logarithmic law layer in comparison to the inner layer increases, which enhances the flow and heat transfer performance in the channel. Reducing the gap size restricts the development of the velocity profile and increases the proportion of the inner layer of the boundary layer in comparison to the channel gap. A smaller gap size weakens the turbulence fluctuation, thus reducing the transverse mixing in the channel gap.

Enhancement technique of heat transfer using inserted twisted tape

Enhancements in heat transfer systems are being applied for recovery of heat in domestic as well as industrial applications in many areas. These are done by generating turbulence and swirl motion in the flow by inserting inserts to reduce the thermal resistance of heat transfer. This paper presents the experimental and numerical results of heat transfer enhancement of cooling of a confined space by inserting twisted tape in a pipe buried in concrete under the floor. The Reynolds number is fixed to 15000. Thirteen cases are considered varying the configurations and designs of twisted tapes and the effects of different types of perforations and slots on twisted tapes are studied. Water as the test fluid flowed in pipe inserted with twisted tapes. Heat is transferred from a heating plate to water along the pipe length through concrete to simulate the heat transfer from a room and the water temperature rise is studied. To validate the experimental results, ANSYS-FLUENT is used as the CFD tool with a Standard k-ε turbulence flow model and the SIMPLE algorithm is implemented. Flow physics behaviour is displayed using velocity and temperature profiles inside the pipe. Insertion of twisted tapes shows that the heat transfer is enhanced due to swirling action, friction and turbulence. The experimental differences in inlet and outlet water temperature validated the CFD simulation results. The temperature distribution on a single twisted tape with increasing twist ratio is found to be the most efficient for heat transfer enhancement of the cooling floor surface with a water temperature rise of 1.62 ºC along a pipe length of 1.8 m.

Optimization of falling film thermosyphons bundle arrangement for large-scale cooling applications by genetic algorithm

Two-phase traditional thermosyphons (TTS) are passive heat exchangers with low thermal resistance for heat transfer. An advanced thermosyphon system named falling-film thermosyphon (FFTS) is developed and introduced in this paper, which eliminates several limits of TTS and improves heat transfer performance. FFTS applications are preferred for large-scale heat transfer applications. In this work, a tube bank of FFTS modules is integrated into a power plant to condense water vapor of the flue gas. Axial- and cross- flow are both analyzed with inline and staggered arrangements. The numerical simulation of vapor condensation in the flue gas is developed, and the results are compared to previously published experimental data. The accuracy of model is validated. The Non-dominated Sorting Genetic Algorithm, version 2 (NSGA-II) is implemented to improve the thermal performance with low material consumption and pressure drop for tube banks. Several parameters of the configuration are optimized. As a result, smaller diameter tubes with longer length can gain better heat transfer performance, however, resulting in a higher pressure drop. The optimized arrangements can improve 60–70% condensation rate and reduce up to 80% material consumption with allowable pressure loss compared to a baseline geometry.

Air-gap diffusion distillation: Theory and experiment

A novel Air-Gap Diffusion Distillation (AGDD) technology operated at normal atmospheric pressure was proposed in this paper. In this plant, cold and hot streams flow counter-currently and the strong hydrophilic porous medium is employed as the hot stream channel to control its flow. The operation characteristics of AGDD were investigated and analyzed theoretically and experimentally. Influences of structure and operation parameters on the performances were discussed deeply. The results showed the water productive flux and Gained Output Ratio (GOR) for the AGDD with one air-gap combination were 0.7244 kg·m−2·h−1 and 1.16 respectively when the thickness and height of air-gap, inlet temperatures of cold and hot streams, mass flow rate of cold stream were 5 mm, 0.9 m, 25 °C, 65 °C and 4 kg·h−1 respectively. When the air-gap height was enlarged to 1.8 m and the mass flow rate of cold stream reduced to 2 kg·h−1, the water productive flux and GOR were 0.5255 kg·m−2·h−1 and 5.01 respectively. The results also indicated that the thermal resistance of heat transfer in the AGDD mainly sourced from the air-gap. The variations of structure or operation parameters did not influence on the desalination rate of AGDD which was higher than 99.8% during experiments.

Research into energy efficiency of the underfloor heating system, assembled dry

The object of research is the thermal parameters of operation of a fragment of the floor heating system assembled dry, under conditions of actual application set in the lab premises. One of the most problematic issues in the course of our experimental study has turned out to be a small area of the investigated heating system, relative to the volume of the room. Considerable ambient air temperature fluctuations resulted in certain difficulties while the heating system entered the quasi-steady mode. We have established in our study the effect of thickness of a heat insulation layer under the heating circuit on a change in the density of heat flow from the floor surface to the air in a heated room. It is noted that the floor heating system, assembled dry, has small thermal inertia due to the absence of a relatively thick layer of the monolithic concrete slab (with high specific heat capacity), which is typically used for the installation of a heating system circuit. Specifically, it was established that the use of ceramic tiles as the finish coating, compared with laminate, significantly reduces the overall thermal resistance of heat transfer from a heat carrier to the air in a heated room. In this case, the presence of an aluminum heat-scattering plate, which is in direct contact with the outer surface of the pipe in a heating circuit, has a positive effect on the uniformity of distribution of thermal field in the plane of the floor. This in turn leads to a reduction in thermal stresses in the finish coating. Calculations show that the quantitative control over thermal load of such a system by changing the consumption of a heat carrier proves to be less effective than the qualitative control through changing its temperature. Experimental studies reveal that the density of a heat flow on the floor surface increases almost two-fold when using ceramic tiles, in comparison with laminate, at all other thermal system settings being almost identical. The research we conducted make it possible to construct a mathematical model for the operation of a floor heating system, assembled dry, whose application would enable the optimization calculations and improvement of the design of a given heater.

Hierarchical graphene foam-based phase change materials with enhanced thermal conductivity and shape stability for efficient solar-to-thermal energy conversion and storage

Recently, graphene foam (GF) with a three-dimensional (3D) interconnected network produced by template-directed chemical vapor deposition (CVD) has been used to prepare composite phase-change materials (PCMs) with enhanced thermal conductivity. However, the pore size of GF is as large as hundreds of micrometers, resulting in a remarkable thermal resistance for heat transfer from the PCM inside the large pores to the GF strut walls. In this study, a novel 3D hierarchical GF (HGF) is obtained by filling the pores of GF with hollow graphene networks. The HGF is then used to prepare a paraffin wax (PW)-based composite PCM. The thermal conductivity of the PW/HGF composite PCM is 87% and 744% higher than that of the PW/GF composite PCM and pure PW, respectively. The PW/HGF composite PCM also exhibits better shape stability than the PW/GF composite PCM, negligible change in the phase-change temperature, a high thermal energy storage density that is 95% of pure PW, good thermal reliability, and chemical stability with cycling for 100 times. More importantly, PW/HGF composite PCM allows light-driven thermal energy storage with a high light-to-thermal energy conversion and storage efficiency, indicating its great potential for applications in solar-energy utilization and storage.

A New Lumped Thermal Resistance Heat Transfer Model for Fuel Pin Structure

The lumped heat transfer methodology is simple, and the solution is very fast, so the lumped parameter approach has been widely used in thermal-hydraulic analysis for fuel pin heat transfer in nuclear reactors. In the conventional lumped thermal analysis of fuel pin structure, each component (such as a pellet, gap, cladding, etc.) is characterized by a concentrated bulk temperature (or averaged temperature), and a bulk thermal resistance. In contrast to this conventional lumped thermal resistance model, in this paper another kind of lumped thermal resistance heat transfer model for fuel pin structure has been developed. In this model, each fuel pin component is still represented by a concentrated lumped mean temperature node, while the location of the mean temperature position of each component is no longer set on the geometrical midpoint center; rather, it is assigned exactly onto the analytical temperature profile. Two thermal resistance elements are assigned for each component in this new model; between each component surface and its associated lumped mean temperature node a thermal resistance is assigned. Heat conduction in the radial direction between the mean temperature nodes of different components is purposely defined to take place at the in-between surface nodes. With this new arrangement, the location of the mean temperature positions for each component can be determined analytically, and all the thermal resistances are redefined, accordingly. The advantage of the presented method is that the temperature profile in the whole pin at any radial position can be reconstructed after a quite easy lumped heat transfer calculation. This advanced methodology can be used in nuclear reactor simulation studies where the fastness of the solution is of concern. It is of great advantage, e.g., for the early prediction of the formation of an internal molten fuel cavity within a fuel pin using this temperature profile, before the lumped pellet average temperature reaches the fuel melting point. This lumped thermal resistance model can be readily used for the sodium-cooled fast reactor design, especially for the optimized design of the pin structure. It can be also extended to the restructured fuel pin through the way that each restructured zone is treated as an individual component, e.g., for taking into account temperature-dependent thermophysical properties.

Efficient on-chip hotspot removal combined solution of thermoelectric cooler and mini-channel heat sink

Hotspot will significantly degrade the reliability and performance of the electronic equipment. The efficient removal of hotspot can make the temperature distribution uniform, and ensure the reliable operation of the electronic equipment. This study proposes a combined solution of thermoelectric cooler (TEC) and mini-channel heat sink to remove the hotspot of the chip in the electronic equipment. Firstly, The TEC's mathematical model is established to assess its work performance under different boundary conditions. Then, the hotspot removal capability of the TEC is discussed for different cooling conditions, which has shown that the combined equipment has better hotspot removal capability compared with others. Finally, A TEC is employed to investigate the hotspot removal capacity of the combined solution, and the results have indicated that it can effectively remove hotspot in the diameter of 0.5 mm, the power density of 600W/cm2 when its working current is 3A and heat transfer thermal resistance is 0 K/W.

Heat transport across a gold nanowire/water interface enhanced by the solution ionic strength

ABSTRACTLithographically fabricated gold nanowires are optically excited with 532nm CW laser and the local temperature change is measured in air, pure water and various concentration aqueous solutions of ionic solutes NaCl, Na2SO4 and MgSO4 using the thermal sensor film of Al0.94Ga0.06N embedded with Er3+ ions. The interface thermal resistance for heat transfer from the excited nanowires into the surrounding liquid is determined from the slopes of the temperature change versus laser intensity plots obtained for the nanowire excitation under various solutions. Addition of ionic solute molecules into the solution decreases the interface thermal resistance and hence leads to increased heat dissipation into the surrounding liquid. Interface thermal resistance decreases exponentially with the ionic strength of solution and saturates around zero for solution ionic strength of 0.3M and higher.

Effect of thermal energy storage in energy consumption required for air conditioning system in office building under the African Mediterranean climate

In the African Mediterranean countries, cooling demand constitutes a large proportion of total electrical demand for office buildings during peak hours. The thermal energy storage systems can be an alternative method to be utilized to reduce and time shift the electrical load of air conditioning from on-peak to off-peak hours. In this study, the Hourly Analysis Program has been used to estimate the cooling load profile for an office building based in Tripoli weather data conditions. Preliminary study was performed in order to define the most suitable operating strategies of ice thermal storage, including partial (load leveling and demand limiting), full storage and conventional A/C system. Then, the mathematical model of heat transfer for external ice storage would be based on the operating strategy which achieves the lowest energy consumption. Results indicate that the largest rate of energy consumption occurs when the conventional system is applied to the building, while the lowest rate of energy consumption is obtained when the partial storage (demand limiting 60%) is applied. Analysis of results shows that the new layer of ice formed on the surface of the existing ice lead to an increase of thermal resistance of heat transfer, which in return decreased cooling capacity.

Experiment Investigation on Heat Transfer Characteristics of a Novel Gravity-Assisted Heat Pipe Heat Exchanger

A novel gravity-assisted heat pipe with fins was developed. The influences of filling ratio, inclination angle and air velocity at condenser section on the heat transfer characteristics were investigated experimentally. The results show that heat power has an important effect on heat transfer characteristics while inclination angle has little effect on that. The minimum heat transfer resistance is got at the filling ratio equaling about 20%. The heat transfer thermal resistance decreases gradually as the air velocity at condenser section increases.

Steady and transient thermo-hydraulic performance of disc with tree-shaped micro-channel networks with and without radial inclination

Steady and transient conjugate heat transfer and fluid flow through tree-shaped radially branching micro-channel networks, embedded in a disk-shaped heat sink, are investigated using numerical simulations. The typical dimensions of the disk are taken as 20 mm radius and 2 mm thickness while the hydraulic diameters of the micro-channel network are in the range of 0.67–0.44 mm, with a constant 0.5 mm height. In steady state, increased levels of complexity of the tree network result in more uniform temperature distribution in the disc ( T ¯ max → T ¯ avg ). However the degree of reduction in temperature reduces for higher complexity. Using the steady state result for minimal flow resistance that yields minimal thermal resistance for heat transfer, the transient response of the disk with differing levels of tree complexities is shown to behave more uniformly under similar imposed heat transfer load. The effect of radial inclination of the flow tree channels on the overall pressure drop of the coolant flow and the disk temperature distribution is also investigated. By allowing 0.1 mm radial inclination to the channel network shows 45 percent reduction in pressure drop for the first level of branching complexity ( k = 1 , n = 3 ) and 25 percent for the highest level ( k = 4 , n = 3 ) as compared to the corresponding planar channel.