Convective Thermal Resistance Research Articles

3D topology optimization design of air natural convection heat transfer fins

This study focuses on the fin-tube heat exchanger and utilizes topology optimization methods to design a completely new fin structure. In this optimization process, complete Navier-Stokes (N-S) equations were used to describe the steady-state incompressible flow, and the Boussinesq model was employed to simulate natural convection. The flow equations were coupled with the heat convection–diffusion equation to achieve topology optimization for natural convection heat transfer. Topology optimization was conducted using density-based optimization methods, and interpolation was performed on the permeability and conductivity of the distributed materials. Given the initial fin structure, an interpolation progressive approach was adopted to obtain a new “ airfoil-shaped” optimized structure through density-based topology optimization method for natural convection. The new structure enhances the convective heat transfer by perforating the fins. The perforations are mainly concentrated in the central region of the heat exchanger and the upper half of the fins. The new structure, compared to the prototype structure, not only has a reduced volume but also exhibits a decrease in convective thermal resistance within a larger range of heat flux densities, as revealed by CFD simulations. Moreover, as the heat flux density increases, the rate of reduction in convective thermal resistance shows an upward trend for the new structure compared to the prototype structure.

Active Thermal Management Method for Improving Current Capability of Power Devices under Influence of Random Convection

The active thermal management (ATM) method can improve the current capability of power devices safely by utilizing the thermal limit of the device. However, the existing methods are not suitable for power devices affected by random convection. This is because the randomness of convection will lead to the uncertainty of increased current capability, which is unacceptable in most engineering applications. To overcome the shortcomings of existing ATM methods, this paper proposed a novel ATM method for power devices under the influence of random convection. In the proposed method, the short-time current capability of the power device is maximized according to the thermal inertia of the device and maximum load current. The continuous current capability of the power device is determined by a maximum continuous current that satisfies the over-temperature risk constraint of the device. To accurately estimate the maximum continuous current, an uncertainty model is presented for the convective thermal resistance of the power device based on wavelet packet transform and Markov chain. A series of experimental studies are carried out by taking a power MOSFET as an example. The experimental results show that the proposed method can safely improve the output performance of the power device without causing random fluctuations in the current capability.

Influence of conjugate thermal boundary on turbulent heat transfer of supercritical CO2 in the horizontal tube

In numerical simulations on horizontal supercritical heat transfer, the tube model excluding or including the solid wall is usually employed. However, a systematic comparison and in-depth elucidation between the non-conjugate and conjugate heat transfer (NCHT, CHT) models are still in absence. To address this critical issue, the present work employs both horizontal models to perform comparative numerical simulations using the shear stress transport (SST) k-ω turbulence model. The results show that the discrepancies in heat transfer between the two modes are negligible under weak buoyancy effect. However, the inconsistencies become more pronounced with intensified buoyancy strength, especially along the top generatrix. Under strong buoyancy effect, the largest deviations in wall temperature and heat flux of the top generatrix between two modes reach up to 396.1°C and 43.5 kW·m-2, respectively, the incorporation of tube wall in CHT computations greatly improved the heat transfer uniformity. The mechanisms that the tube wall acts as a “damper” regulating the heat flow are revealed, which is mainly via the circumferential heat transport and spread from the top where the fluid convection thermal resistance augments; whereas the exclusion of the solid tube wall results in the loss of the surface heat flux adjustment, and the top heat transfer degradation progressively worsens then the wall temperatures sharply rise. As the buoyancy level is within the range of BuP, max<85.5, the heat migration from the top within the tube wall of CHT simulations is less than 17.8%, indicating the simplified NCHT model can be effectively used for sCO2 heat transfer simulations of the studied horizontal tube with acceptable accuracy.

Machine Learning Assisted Microchannel Geometric Optimization—A Case Study of Channel Designs

At present, microchannel heat exchangers are widely applied in the fields of air-conditioning and heat pumping applications given their high heat transfer performance, compact size, and low material cost. However, designing and optimizing the channel geometries remain challenging, as they require balancing multiple competing objectives to achieve the optimal performance. This study investigates various parameters, including the channel count, wetted perimeter, cross-sectional area, and mass flow rate for each channel, to achieve the optimal efficiency. The optimization objectives include maximizing the heat transfer rate, minimizing the refrigerant convective thermal resistance, maximizing the refrigerant heat transfer coefficient, and minimizing the pressure drop. A multi-objective genetic optimization algorithm, in conjunction with artificial neural network (ANN)-based machine learning models, was used to predict the heat transfer rate to speed up the calculation process during the optimization. We identified that a gradient reduction in the wetted perimeter from the air inlet along the airflow direction could enhance the heat transfer rate. Additionally, the results indicate that an increase in the number of channels leads to an enhanced heat transfer efficiency rate. However, with the increase in the number of channels, the cross-sectional area of each channel is correspondingly reduced to maintain a consistent overall cross-sectional area. This reduction increases the fluid resistance, leading to an increased pressure drop across the system. This observation is critical for a microchannel design optimization, highlighting the importance of attaining a balance between achieving a higher heat transfer efficiency and maintaining a favorable fluid dynamic performance.

Thermal Resistance of Heated Superhydrophobic Channels With Thermocapillary Stress

Abstract A pressure-driven channel flow between a longitudinally ridged superhydrophobic surface (SHS) and solid wall is studied, where a constant heat flux enters the channel from either the SHS or solid wall. First, a model is developed which neglects thermocapillary stresses (TCS) in the transverse direction. The caloric, convective, and total thermal resistance are evaluated, and their dependence on the shape of the liquid–gas interface (meniscus), gas ridge width, texture period, channel height, streamwise TCS, Péclet number, and channel length is established. The caloric resistance is minimized with menisci that protrude into the gas cavity, large slip fractions, small channel heights, and small streamwise TCSs. When heating from the SHS, the convective resistance increases, and therefore, a design compromise exists between caloric and convective resistances. However, when heating from the solid wall, the convective resistance remains the same and SHSs that minimize caloric resistance are optimal. We investigate both water and Galinstan for microchannel applications and find that both configurations can have a lower total thermal resistance than a smooth-walled channel. Heating from the solid wall is shown to always have the lowest total thermal resistance. Numerical simulations are used to analyze the effect of transverse TCSs. Our model captures much of the physics in heated superhydrophobic channels but is computationally inexpensive when compared to the numerical simulations.

A numerical investigation of hydrogen impingement-effusion array jet for a heat sink cooling using solid/porous fins: A thermo-hydrodynamic analysis

Increasing the aim of hydrogen use as fuel in vehicles motivates applying it to other critical requirements, such as cooling, owing to its thermal properties. Therefore, the effect of applying hydrogen impingement jets for cooling a hot surface of electrical devices, such as a prismatic battery, is investigated in this numerical study. The dependent variables are the volumetric flow rate, the jet distance from the target to the jet diameter, the coolant type, and the fins configuration, called in-line and staggered, and types, as solid or porous. The convective heat transfer coefficient, thermal resistance, pressure drop, and temperature and velocity distribution for the studied cases are considered. The simulation is conducted using the SST k-ω turbulence model, SIMPLE algorithm, and second-order method for discretization differential equations. Applying hydrogen as the coolant decreases the maximum temperature up to 2% and increases heat transfer by about 10%, while it mitigates the pressure drop by a factor of 1/12 if compared to air coolant. Further, inserting porous fins improves heat transfer by about 10% and 17%, respectively for air and hydrogen coolant in comparison with the case without fins. The fin decreases the temperature on the root and makes the recirculation zones.

Power Cable Terminal Fault Detection Method Based on Infrared Thermal Imaging

According to the analysis results, most conventional power cable terminal fault detection methods use the time-domain joint analysis principle to determine whether the terminal has abnormal and fault-hidden dangers. Then, it carries out fault detection. The detection process is complex and time-consuming, and the cable terminal fault information cannot be obtained the first time. Based on this, infrared thermal imaging technology is introduced, and a new fault detection method is proposed. The method comprises the following steps. Firstly, acquire operation data of a power cable terminal and preliminarily judge whether the cable terminal is abnormal according to dynamic changes of conductive thermal resistance and convective thermal resistance of the terminal. Secondly, introduce an infrared thermal imaging technology on the basis and capture infrared rays radiated by the power cable terminal. Lastly, acquire temperature information of the surface of the terminal, combine the temperature with infrared ray quantity results in two directions, and make all-round detection of cable terminal faults. The experimental analysis indicates that the proposed cable terminal fault detection method can complete the fault detection in a shorter time. The detection time is shorter, and the efficiency is higher.

Investigations on Five PMMA Closed Types of Piezo Actuators as a Cooling Fan

There are five closed types of piezo actuators (closed type of PA, closed PA) as a cooling fan relative to those different PAJs of the previous work (open type of PAJ, open PAJ) for analysis in the present study. Closed PA was composed of circular piezoelectric ceramics (PCs) and acrylic (PMMA) plates and investigated on five different types at operating conditions. The results show that the noise of the closed PA is quieter than that of the open PAJ by about 10 dB. When the closed PA is deposed at a suitable distance of 10 to 20 mm from the heat source, averting sucking back the high-temperature fluids around that, the thermal convection coefficient is above 120% more than that of the conventional rotary fan. The cooling performances of these five closed PAs were evaluated by thermal analysis technique, and the convection thermal resistance of the best closed PA can be decreased by over 15%. In terms of energy consumption, a monolithic closed PA was less than 10% than that of a rotary fan. Among these five closed PAs, the best one has the essential qualities that the diameter of the piezoelectric sheet is 41 mm, the opening length is 4 mm, and the outer opening length is 10 mm. Moreover, the best operating conditions are a voltage frequency of 300 Hz and a release distance of 15 mm in the present study.

FLOW AND HEAT TRANSFER ANALYSIS OF MICROCHANNEL HEAT SINK WITH SOLID INSERTS FOR ELECTRONIC COOLING APPLICATIONS

The emerging field of electronic industry requires compact electronic components without sacrificing the reliability and performance of the equipment. The microchannel heat sink is the efficient liquid cooling technique to dissipate large heat flux in a miniature electronic component. In this study, the improvement in hydrothermal performance of conventional plane microchannel heat sink is analyzed by introducing different solid inserts in the channel flow path. A three-dimensional thin-walled rectangular microchannel with fixed aspect ratio and hydraulic diameter is considered for numerical analysis. The study aims to enhance the heat transfer by breaking the thermal boundary layer and decreasing the convective thermal resistance with small increase in pumping power. The coolant and solid insert used in the study are water and aluminum, respectively. Finite volume method-based solver ANSYS Fluent is used for simulating all cases. The effects of different inserts on fluid flow and heat transfer characteristics are analyzed for the Re number range of 500 &amp;le; Re &amp;le; 1000. The entropy generation minimization principle and performance index are the methods used for analysis. Based on the combined effect of flow and thermal characteristics, the insert with converging rectangular geometry gives the overall best performance. The optimum insert is further modified by varying the convergent angle in order to achieve a maximum possible heat transfer enhancement.

Measuring convection heat transfer coefficients and thermal resistance for protective fabrics using a heated cylinder in a wind tunnel

One tradeoff in design of protective clothing for firefighters and other workers is between thermal protection and thermal stress management. Standard methods for assessing thermal stress management of protective clothing include guarded hot plate and mannequin tests. In this study, a fabric-covered heated cylinder was used in a wind tunnel to measure convection heat transfer coefficients and thermal resistance at various air speeds. Measurements demonstrated the impact of air permeability and wind speed on these parameters. When the cylinder was covered with low permeability fabrics, convection heat transfer coefficients were similar to values predicted using a correlation for bare cylinders. Thermal resistance measurements generally ranked fabrics in the same order as guarded hot plate tests. This cylinder test is more representative of the body’s geometry than hot plate tests, but less expensive to conduct than mannequin tests, and could serve as a bridge between these two tests for design purposes.

Investigation of thermal and hydraulic performance of MEMS heat sinks with zig-zag microchannels

• Zig-zag microchannels enhance performance of MEMS heat sinks. • Studies conducted for Reynolds number from 250 to 1500. • Lower thermal resistance with zig-zag microchannel compared with straight microchannels. • Higher pumping power with zig-zag microchannel compared with straight microchannel. This work carries out an investigation on the performance of a MEMS microchannel heatsink with multiple zig-zag microchannels (MCHS zz ). The heat sink is realized in silicon and uses water as the coolant. The study is conducted by employing Fluent module of Ansys Workbench. The performance of zig-zag and straight MCHS are compared for Reynolds number ( Re ) from 250 to 1500. The performance of the MCHS is quantified in terms of total thermal resistance ( R th ) and pumping power ( PP f ) where R th of zig-zag MCHS is better than that with straight microchannel; the enhanced thermal performance is achieved at the cost of increased PP f . Based on the cases studied in this work, it is observed that the relative change in R th varied from 26% to 51% with increase in Re and the corresponding relative change in PP f varied from 40% to 120% with increase in Re . By employing zig-zag microchannels, it is possible to significantly reduce convective thermal resistance ( R th,cov ), for a particular Re . Additionally, the influence of microchannel hydraulic diameter, orientation of microchannel, number of repeating units, and microchannel spacing are studied in this work. It is identified that the R th and PP f over the considered range of Re decreased and increased, respectively, regardless of the geometric parameters. Additionally, R th decreased with increase in geometric parameters for a specific Reynolds value until a threshold value beyond which no changes occur.

On the assessment of the thermal performance of microchannel heat sink with nanofluid

By increasing demands for high thermal performance and energy efficiency, attentions to microchannel heat sinks (MCHSs) as the suitable method for heat flux dissipation from thermal systems have increased significantly. Microchannel heat sinks can be widely employed in electronic devices for higher heat removal rate and to provide best performance and durability for electronic systems. The critical issue associated with MCHSs is their ability for integration of effective thermal performance. In this work, on the assessment of the thermal performance of microchannel heat sink with nanofluid is experimentally examined. The heat removal performance of the pure water and nanofluid through the MCHS is studied. Different important parameters, such as dimensionless wall temperature, pressure drop, mean convection heat transfer coefficient, thermal resistance, and uniformity index, are investigated. The results indicated that the maximum suppression value of the thermal resistance attained by employing the nanofluid is 12.61%. The uniformity index of the heating surface is increased as the Re number increases. More suppression in the wall temperature can be observed as the volume concentration of nanoparticles is increased. By increasing the total flow rate and using the nanofluid, the hot spots on the heating surface are suppressed. Finally, the maximum gain value of the nanofluid for the mean convection heat transfer coefficient is up to 14.43%.

Analysis on the heat transfer performance of supercritical liquified natural gas in horizontal tubes during regasification process

The submerged combustion vaporizer (SCV) is an indispensable peak-shaving heat exchanger in the regasification process of liquified natural gas (LNG). This paper mainly studies the trans-critical heat transfer performance of LNG in the horizontal tube of SCV under the convective heating condition. To this end, the conditions of fluid temperature, mass flux, and tube diameter are comprehensively investigated through numerical simulation. Based on the simulation results, the influence of the buoyancy on total and local heat transfer performance is analyzed from the distribution of flow field and thermal-physical property in the boundary layer. The results indicate that the buoyancy leads to a significant difference in convection thermal resistance around the circumference. Tube diameter and mass flux affect the degree of heat transfer strengthening in the lower part and heat transfer deterioration in the upper part of the tube, as well as the occupied circumferential proportion of the corresponding region. Considering the effects of local heat transfer strengthening and deterioration, buoyancy shows an enhancement effect overall on the heat transfer process, especially under the conditions of low mass flux and large diameter. Based on the mechanism analysis of the convective heat transfer, two correlations are proposed to calculate the local convection thermal resistance in the buoyancy strengthening and buoyancy deterioration region, respectively.

Design and Testing of a Bearing less Piezo Jet Micro Heat Sink

The present study utilized piezoelectric ceramics (PC) as actuators to design five new piezoelectric heatsinks and preliminarily investigated the different design types and their operating conditions, such as the frequency, placement distance, thickness between piezoelectric sheets, piezoelectric sheet size, and noise produced. The results showed that when the micro heatsink bearingless piezo jet was placed too close to the heat source, the high temperature sucked back the surrounding fluids, causing the fluid chamber temperature to rise and the cooling effect to be reduced. Therefore, the heatsink should be placed between 10 and 20 mm from the heat source. With the proper distance, the heat convection coefficient was 200% greater than that of a traditional rotary fan. The cooling effect of the five heatsinks was calculated using the thermal analysis method, and the results indicated that the convection thermal resistance of the best heatsink could be reduced by about 36%, and the frequency, flow velocity, and noise were all positively correlated. When the supplied piezoelectric frequency was 300 Hz, the noise level was similar to that of a commercial rotary fan. The tested heatsinks had one of two volumes depending on the size of the piezoelectric sheet, including 3150 mm3 or 4050 mm3, respectively. An array of 25 to 32 micro heatsinks of the same size were connected in series. The power consumption of any single heatsink was 10% of that for a rotary fan. Among the five types of heatsinks, the best type had a piezoelectric sheet diameter of 41 mm, a piezoelectric thickness of 2 mm, and an opening length of 4 mm. Furthermore, the best operating conditions were found at a frequency of 300 Hz and a placement distance of 20 mm.

Effect of freezing rate on the quality of frozen strawberries (Fragaria x ananassa)

Domestic blast freezers have been designed to achieve fast freezing rates to preserve the properties of food. However, the relationship between the refrigeration system parameters and the food quality is not yet fully understood. The aim of this study was to investigate the effect of the refrigeration system operating conditions not only on the freezing rate but also on the food quality. To this end, an experimental apparatus capable of controlling the air temperature and velocity was designed and constructed. The freezing rate, drip loss and hardness of strawberry samples (Fragaria x ananassa) frozen applying different air temperatures and velocities were measured. Also, a mathematical model was implemented to predict the freezing rate. On comparing the numerical and experimental results, differences within a ±20 % error band were observed. In addition, the experimental results show a nonlinear relation between the operating conditions and the freezing rate, which was explained by the unbalanced convective and conductive thermal resistances during the freezing process. Regarding the quality of the strawberry samples, the hardness increased by 73 % and the drip loss reduced to around 5.5 % of its original value when the operating conditions were changed from -20°C / 0.4 m⋅s−1 (typical of ordinary household freezers) to -53°C / 6.5 m⋅s−1. Finally, a regression analysis was carried out to predict the drip loss as a function of the operating conditions, resulting in an exponential relation with R2 = 0.92.

Real-time thermal error prediction and compensation of ball screw feed systems via model order reduction and hybrid boundary condition update

Thermal deformation of a ball screw is a quasi-static process and incurs positioning error. To obtain a rapid prediction model to compensate for this deformation error, a reduced-order finite element (FE) model is required. For updating the boundary conditions (BCs) of such a reduced-order model (ROM), a hybrid BC update process composed of the response surface method (RSM) and 3-point temperature measurement is devised in this paper. We develop an adaptive reduced-order model (AROM) for real-time prediction and compensation of the error in a time-varying environment under any operating conditions. This system solves not only the heat generation rate (HGR) and nut movement effects on the lubricant temperature variation, but also the convective heat transfer coefficient (CHTC) and thermal resistance (TR) effects at joint interfaces. High prediction accuracy has been verified via a test-bed with a precision linear scale. An AROM server application is also proposed for a thermal error free smart factory.

Machine learning enabled condensation heat transfer measurement

Measuring condensation heat transfer and its associated heat transfer coefficient is not trivial. Rigorous measurements require careful experimental design and tradeoff studies to properly select sensor type, sample geometry and size, coolant fluid and flow rate, operating conditions, working fluid purity, purge methodology, and measurement protocol. Conventional tube-based condensation heat transfer measurements quantify the change in the enthalpy of a single-phase coolant flow via measurement of the inlet and outlet bulk coolant temperatures. The uncertainties associated with this classical and well-established experimental method are high. The high uncertainty stems from the high characteristic heat transfer coefficient or heat flux associated with the condensation process, making the thermal resistance on the external tube side typically on the same order of magnitude as the internal single-phase coolant convective heat transfer thermal resistance. Even when taking the utmost care and using extremely accurate sensors having low uncertainty, the relative uncertainties of heat flux and heat transfer coefficient can be in the range of 20% to 100%. Here, we take advantage of machine learning (ML) to develop an optical visualization method for dropwise condensation heat transfer characterization. Using state-of-the-art intelligent vision, we demonstrate a previously-unexplored method for characterizing the condensate droplet shedding frequency, droplet shedding size, and heat flux without the need for high-speed imaging. We verify our technique by conducting rigorous steam condensation measurements on Parylene C coated smooth copper tube samples having 500 nm, 1 μm, and 5 μm Parylene C thicknesses. We validate our ML predictions with data obtained simultaneously using the enthalpy-change method on a custom and well-established condensation chamber. In contrast to conventional heat transfer measurement methods, the uncertainty of our ML method is constant (∼10%) and does not vary with heat flux. We finally demonstrate the key advantage of our ML measurement technique on a custom-made tube having axially varying surface properties resulting in differing local heat transfer coefficient. Our ML heat transfer measurement method enables the high fidelity characterization of phase change heat flux, reduction in relative measurement uncertainty, resolution of local effects, and elimination of the need for temperature measurement across samples.

Research on the speed‐dependence of heat convection in high‐speed EV motors and its application in the model‐based rotor temperature monitoring technology

Using the embedded onboard digital thermal model is an economical and effective way to monitor the rotor temperatures of the electric vehicle (EV) propulsion Permanent Magnet Synchronous Motor (PMSM) in real-time. The onboard thermal model is generally in an analytical form with constant thermal resistances. However, in practical situations, the thermal convection intensities on the surfaces of the air gap and the end-cavity vary significantly with the rotational speed, producing a non-negligible influence on the estimation accuracy of the onboard thermal model. Aiming at eliminating this error and improving the accuracy, this paper explores the variation of the airflow's aerodynamics characteristics and the heat convection with the rotational speed through a syncretic study of theoretical, experimental, and numerical methods. It is found that the changing trend of the airflow presents a multi-stage characteristic in the speed range of 0–12,000 rpm. The speed-dependent convective thermal resistances are formulaically parameterised and then used to replace the related constant thermal resistances in the thermal model. The standard vehicle driving cycle test result shows that this optimisation brings a nearly 11% reduction of the overall estimation error.

Experimental and numerical investigation on thermal and hydraulic performance of novel serpentine minichannel heat sink for liquid CPU cooling

A novel serpentine design for minichannel heat sink is investigated experimentally and numerically to enhance thermal performance of the heat sink for liquid CPU cooling. The heat sink is fabricated from aluminum and is located in a plexiglass container. The channels have square cross section with deep and width of 2 mm. Mass flow rate and inlet temperature effect on the performance of the maze serpentine heat sink are studied. Five different mass flow rate varies between 0.0017 kg/s and 0.0087 kg/s and three different inlet temperature (15°C, 20°C and 25 °C) is investigated. Pure water is used as the coolant. The results indicate that by changing the minichannel design from straight to the proposed mazed shape geometry, baseplate temperature decreases 11.2% and Nusselt number increases 4.2 times in the maximum mass flow rate. Thermal resistance is decreased in the novel maze serpentine minichannel heat sink and by increasing mass flow rate the difference get larger so that maximum convective thermal resistance difference for and the novel maze serpentine minichannel is 54% that is observed in the lowest mass flow rate. Also, higher thermal performance is achieved in the maze serpentine minichannel heat sink.

Modeling Method for Thermal Field of Turbulent Cooling Dry-Type On-Board Traction Transformer in EMUs

Adopting dry-type on-board traction transformer (OBTT) is one of the main ways to reduce the weight of electric traction systems, and its thermal characteristics are crucial for the safe operation of high-speed electric multiple units (EMUs). However, the existing thermal modeling methods either cannot realize the fast calculation of thermal characteristics of dry-type OBTT, or cannot accurately obtain the turbulence heat-transfer intensity when using the high-speed train-induced wind cooling. This article proposes an efficient method for dry-type OBTT thermal characteristics calculation. First, the thermal network of the winding area is proposed considering the thermal developing region of the air duct and the heat flow in the insulation paper between conductors. Then, the effects of key parameters on convective heat-transfer intensity are obtained by computational fluid dynamics (CFD) parametric sweeps, and then a novel local Nusselt number correlation for accurately calculating the turbulent convective thermal resistance is proposed. Finally, validation is performed with an experimental setup. The experimental results show that the computationally efficient thermal network model proposed in this article can quickly obtain the winding temperature distribution and the location of the hot-spot temperature. Moreover, the thermal analysis of a 6.3 MVA dry-type OBTT was conducted using the proposed modeling method.