Jet Heat Research Articles

Convective heat transfer of microchannel heat sinks with pinfins and jets for a heat flux up to 500 W/cm2

This study proposes a kind of jet impingement microchannel heat sink with pinfins and then investigates the effects of jets arragement, pinfins placement angle, and pinfins shape on the flow characteristics and heat transfer of various heat sinks. It is found that the arrangement of jets has a minor influence on the flow pressure drop but has a significant influence on the heat transfer. Considering the heat transfer coefficient and pressure drop, the 5 × 6 array jet heat sink performs the best. The shape and placement angle of pinfins greatly affect the flow characteristics and heat transfer. Elliptical fins have a more advantageous influence on the overall performance of the heat sink compared with rectangular fins. The angle of the elliptical fins corresponding to maximum thermal performance factor value decreases with Reynolds number, the trend not seen in rectangular fins. The maximum thermal performance factor of heat sink, with rectangular fins at the angle of 15° is increase by 6.73 %, and it is increased by 7.86 % for the case with elliptical fins at the angle of 30°. The numerical simulations of the present design are justified by the analysis from the perspectives of entropy generation and Field Synergy Principle.

Hybrid heating in the fused filament fabrication process

AbstractAltering the heating regime of the polymer during the fused filament fabrication (FFF) process can lead to changes in both the behaviour of the polymer and the characteristics of the printed product. This study proposes replacing the traditional resistive heating system with two hybrid systems that introduce an additional temperature of 120–160 °C: one combining resistive and hot air jet heating, and the other combining resistive and infrared radiation heating. The samples printed using these hybrid systems were analysed using differential scanning calorimetry (DSC) and visually inspected. Commercial ABS and PLA filaments were used in the experimental programme. A model to evaluate the polymer’s melting during the printing process was proposed and experimentally validated. Visual testing revealed that the printed lattice structure had smaller voids, characterised by depositions that were flattened rather than circular in cross-section due to the extended time in a viscous/partially molten state. The elongation viscosity and storage modulus decreased by approximately 10%, with a slightly smaller decrease observed for the infrared radiation heat source. The glass transition temperature remained unchanged, and the molecular mobility was not affected by the additional heat. Similarly, the energy required for crystal formation was unaffected by the supplementary heat. The mechanical behaviour of the printed pieces during compression tests was also influenced by the addition of a second heat source. For both materials, a decrease in deformability was observed as the temperature of the hot air jet increased.

Study on smoke sealing effectiveness of the air curtain in exit bifurcated tunnel fires

The feasibility of using a smoke prevention air curtain in the exit bifurcated tunnel was investigated through 1:10 scale model experiments. The results indicated that the jet velocity and heat release rate (HRR) significantly influenced the smoke sealing effectiveness of the air curtain. However, once the jet velocity reached a certain extent, the longitudinal fire location had a relatively limited effect on the smoke sealing effectiveness. Subsequently, numerical simulations were conducted to investigate the influence of the HRR, jet velocity, longitudinal velocity, and bifurcation angle on the smoke sealing efficiency of the air curtain. The simulations were based on the evacuation strategy of bifurcated tunnel under longitudinal ventilation, assuming no smoke backflow upstream of the fire. The findings revealed that the smoke sealing efficiency was positively correlated with the jet velocity, negatively correlated with the HRR and longitudinal velocity, and somewhat increased with the increase of bifurcation angle. Additionally, a prediction model was proposed for determining the optimal air curtain jet velocity in the bifurcated tunnel. This model was developed using dimensional analysis principles and taking into account various influencing factors, which can provide valuable insights for designing smoke control measures and enhancing safety in bifurcated tunnel fires.

Numerical and experimental study on manifold-distributed jet microchannel with micro-pin-fins

Traditional parallel microchannels are inadequate for managing the thermal requirements of newer, large-area and high-heat-flux chips. This inadequacy arises from its high flow resistance and poor temperature uniformity. Therefore, A novel manifold microchannel heat sink combining manifold inlet/outlet structures, distributed array jets, and micro pin-fins is introduced. Numerical simulations and experimental methods were employed to investigate the flow and heat transfer performance of the heat sink. Experimental results show that, with an average heat flux density exceeding 330 W/cm2 and a total power of 2500 W, the average chip temperature remains below 70 °C, ensuring efficient heat dissipation. Secondary impingement occurs at sufficiently low jet chamber heights, resulting in an increase of 3 times in the average Nusselt number for a smooth base plate. For the micro-pin-finned surface, it is shown that the presence of micro-pin-fins is not necessarily conducive to heat transfer enhancement. A critical volume fraction of micro-pin-fins (critical fin-ratio) exists where the obstructive effect and enhancement effect balance each other out. This value is influenced by multiple parameters. Finally, a novel relationship for predicting the average Nusselt number of jet impingement was proposed, with an average absolute error of less than 15 %. This series of investigations addresses the existing gaps in research on manifold-distributed array jet heat sinks and provides a meticulous theoretical foundation for the design and optimization of heat sinks facing the high-power chips.

Modeling of workpiece temperature suppression in high-speed dry milling of UD-CF/PEEK considering heat partition and jet heat transfer

Modeling of workpiece temperature suppression in high-speed dry milling of UD-CF/PEEK considering heat partition and jet heat transfer

Prediction of heat transfer for a single round jet impingement using the GEKO turbulence model

Two-dimensional Reynolds-Averaged Navier–Stokes simulations are performed to study a single, round impinging jet heat transfer problem, utilizing the generalized k-omega (GEKO) turbulence model as a benchmark. The simulations are performed at a jet Reynolds number of 23,300 and a nozzle-to-plate distance of 2.0 where a second peak in surface Nusselt number is observed. The effects of the three primary (Csep, Cmix and Cnw) and three auxiliary (Cbf,l, Cbf,t and Cnw,sub) GEKO calibration parameters are investigated. The results indicate that Cmix has the most significant impact on the laminar-turbulent transition zone. A deep learning based regression model is developed and trained using the simulation outputs for fast predictions of the heat transfer curve. Using Csep=1.1, Cmix=−0.7, Cnw=2.0, Cnw,sub=2.25 and Cbf,t=3.0 along with laminar-to-turbulent transitional modeling values, provides the best agreement with experimental results from previous studies.

Optimisation Design of Thermal Test System for Metal Fibre Surface Combustion Structure

The metal fibre surface combustion structure has the characteristics of strong thermal matching ability, short response time, and strong shape adaptability. It has more advantages in the thermal test of complex hypersonic vehicle surface inlet, leading edge, etc. In this paper, a method of aerodynamic thermal simulation test based on metal fibre surface combustion is proposed. The aim of the study was to create a uniform target heat flow on the inner wall surface of a cylindrical specimen by matching the gas jet flow rate and the geometry of the combustion surface. The research adopted the optimisation design method based on the surrogate model to establish the numerical calculation model of a metal fibre combustion jet heating cylinder specimen. One hundred sample points were obtained through Latin hypercube sampling, and a database of design parameters and heat flux was established through numerical simulation. The kriging surrogate model and the non-dominated sequencing genetic optimisation algorithm with elite strategy were adopted. A bi-objective optimisation design was carried out with the optimisation objective of the coincidence between the predicted and the target heat flux on the inner wall of the specimen. The results showed that the average relative errors of heat flow density on the specimen surface were 8.8% and 6% through the leave-one-out cross-validation strategy and the validation of six test sample points, respectively. The relative error values in most regions were within 5%, which indicates that the established kriging surrogate model has high prediction accuracy. Under the optimal solution conditions, the numerical calculation results of the heat flow on the inner wall of the specimen were in good agreement with the target heat flow values, with an average relative error of less than 5% and a maximum value of less than 8%. These results show that the optimisation design method based on the kriging surrogate model can effectively match the thermal test parameters of metal fibre combustion structures.

Experimental investigation of the structure effects on the energy recovery and jet impinging process in a fast cooling Joule-Thomson cryocooler

Compared to other Joule-Thomson (J-T) refrigeration systems, open-cycle miniature J-T cryocoolers offer exceptional rapid cooling capabilities, making them ideal for applications such as infrared guidance in missiles. The energy recovery in the heat exchanger enables the refrigerant reach saturation temperature quickly and improve the jet liquefaction rate. The heat transfer intensity of the impinging jet determines the cooling rate of the target. Hence, heat recovery and the impact jet process are the primary factors behind this rapid cooling, with distinct roles that require separate consideration. The structural differences will directly affect the energy recovery and jet impact process. To investigate these affects, an experimental system for rapid cooling J-T cryocoolers was established and three distinct cryocoolers with substantial structural variations were designed. The important structures, including jet height, orifice diameter, enhanced heat transfer treatment of the cold plate, heat exchanger height, and heat exchanger cone angle, were closely studied. In the range of our experiments, it was found that larger heat exchanger cone angle leading better energy recovery performance, while the length of the heat exchanger is limited by the type of refrigerant. Longer heat exchanger actually introduce too much thermal mass for the refrigerant with better energy recovery performance. In the aspect of jet impingement, enhanced heat transfer treatment and larger jet height will improve the jet heat transfer intensity.

Self-similarity of obliquely impinging jet heat flux fields in a range of Mach numbers from 0.1 to 1.0

This study reveals self-similarity (Mach-number-invariance) of the normalized time-averaged surface heat flux fields measured using temperature sensitive paint (TSP) in an obliquely impinging jet in a range of the nozzle exit Mach numbers from 0.1 to 1.0. Further, this self-similarity is also found in the normalized root-mean-square (RMS) fields of the heat flux fluctuation and the normalized skin friction fields extracted from the surface temperature and heat flux fields. To elucidate the origin of this self-similarity, a semi-empirical model is proposed for the maximum surface temperature difference used for normalization, indicating that the Mach number effect on the impinging jet heat transfer is mainly represented by the recovery temperature at the impingement point.

Experimental study of convective heat transfer distribution of non-interacting wall and perpendicular air jet impingement cooling on flat surface

An experimental study evaluated heat transfer with perpendicular and wall-impinging air jets on stainless steel foil, for Reynolds numbers Re = 3000, 5000, 8000, and 10000, where the perpendicular jet targets the bottom and the wall jet the top, creating a unique, non-interacting effect. Distances to nozzle diameter ratios for wall jets (S/d = 4, 6, 8, 10) and perpendicular jets (Z/d = 2, 4, 6, 8) were varied. Significant heat transfer increases were noted, with the Nusselt number rising by up to 49.20 % for a Z/d = 6 and S/d = 8 combination at Re = 5000. Improvements ranged from 10.03 % to 49.20 %, peaking when the jets' high heat transfer regions overlapped. Optimal performance for Re = 3000 was at S/d = 10, aligning the wall jet's maximum with the perpendicular jet's stagnation area. For Re = 5000 to 10000, optimal S/d values were 8 and 4 for Z/d = 6, 8 and Z/d = 2, 4, respectively. The Nusselt number increase ranged from 29.21 % to 46.57 % at S/d = 10 for Re = 3000, the highest among all tested values. Wall jet heat transfer downstream increased by 90–105 % over perpendicular jets in corresponding regions. Increasing the wall to perpendicular jet distance improved heat transfer near the stagnation point, suggesting this cooling method for high-density electronics like CPUs and GPUs.

Lorentz force and solar energy case study on CNTs and pollytetrafluoroethylene (PTFE) paraffin oil-based hybrid nanofluid flow through a porous divergent/convergent channel

The concept of thermal transfer has fundamental implications in many areas, from the most basic physiological processes to the most complex technological systems. Kerosene oil, often known as paraffin oil, is frequently used as a fuel for jet lighting, engines and heating processes in many technologies. The rate of heat transmission is the fundamental requirement of all phenomena. But this cannot be achieved from base fluid like paraffin oil due to its less efficiency to rise the rate of heat transmission and save energy lost due to high temperatures. Therefore, current analysis is to explore effect of the Lorentz force, viscous dissipation and solar radiation on kerosene oil with the combination of single-walled carbon nanotubes (SWCNTs) and Polytetrafluoroethylene (PTFE) in a porous convergent/divergent and stretching/shrinking channel. SWCNTs and PTFE are added to kerosene oil to produce the hybrid nanofluid. The designed problem is modeled in terms of partial differential equations PDEs. The nonlinear PDEs system is then reduced into ODEs using the transformations framework. The ND Solve technique is applied to numerically simulate the reduced boundary value problem, and the results are sketched and discussed. Obtained results demonstrate that velocity profile increase for the divergent channel and a fall in the convergence channel against higher value of porosity parameter and as well as Hartmann number.

Effect of mechanical stirring on heat transfer and flow of crude oil in storage tanks under different heating methods

Mechanical stirring can promote uniform distribution of crude oil temperature and increase the heating rate, but the effect is different for different heating methods in crude oil storage tank. In this paper, jet heating and tubular heating are used as the heating methods in the tank, the physical and mathematical models under the synergy of two heating methods and mechanical stirring are established, FVM and SIMPLE algorithm are used to numerical calculate. The results show that mechanical stirring can enhance the flow of crude oil in the tank, affect greatly the temperature and velocity fields of crude oil and improve uniformity of the temperature field. Mechanical stirring has more significant effect on tubular heating for the 1000 m3 actual oil storage tank. For tubular heating, mechanical stirring can increase the crude oil average velocity 13.8 times, heating rate by 0.123 °C/h, heating efficiency by 8.37 % and reduce the temperature variance by 92.03 % during 2h of heating. At the same time, the indicators of economic benefits for different heating methods are put forward, and the order of energy consumption per unit temperature rise is obtained: Tubular heating > Jet heating > Jet heating and stirring > Tubular heating and stirring.

Analysis of Confined Jet Impingement in Converging Annular Microchannel Heat Sinks

Jet impingement cooling is deemed an excellent choice for the thermal management of high-power electronics. However, high-pressure drop penalties and low local heat transfer coefficients in regions far from the jet zone are its drawbacks. Although it is reported that recirculation areas appear because of the entrainment, the effects of recirculation size on thermal behavior are not understood well enough. Here, jet impingement heat sinks with converging annular channels are employed in a numerical investigation to minimize the adverse cooling effects associated with an impinging jet in a microchannel. The realizable k−ε turbulent model is used for modeling thermal and turbulent flow fields (Re=5,000 to 25,000). It was found that the different flow recirculation zones in small scales are responsible for the enhanced heat transfer rate. While the thermal performance of a converging wall jet impingement heat sink is higher than its flat wall counterpart at low Re numbers, the thermal performance results are in favor of the flat wall jet impingement heat sink at high Re numbers. The flow recirculation area shrinks in converging channels at high Re numbers, thereby deteriorating the thermal performance of the converging channel compared with a flat wall jet heat sink. Also, it was found that employing steeper converging channels shrinks the flow recirculation region, resulting in up to 59% lower pressure drops at Re=25,000. The present study examines the role of flow recirculation at different Re numbers on the thermohydraulic performance of jet impingement converging annular heat sinks.

Numerical simulation of thermodynamic response law of chemical pipeline corridor under jet fire environment

To accurately predict the relationship between temperature rise of chemical corridor pipelines and time under a jet fire environment, a chemical corridor in Shanghai is taken as the research object, and a model of chemical corridor pipeline is constructed with FDS software to study the thermodynamic response law of the petrochemical pipeline under the action of jet fire. The results show that the temperature rise rate of the petrochemical pipeline increases significantly with the increase of the jet caliber, and the heat radiation intensity generated by the jet fire increases first and then decreases with the increase of the horizontal distance from the nozzle. The wind speed and direction affect the cover of the jet fire, and then affect the temperature rise rate and peak temperature of the pipeline. Some variables were selected as the factors affecting the temperature rise of chemical pipe corridor, including jet fire duration, flame combustion heat, combustion efficiency, jet heat radiation intensity, distance between pipeline and leakage port, injection fire length, wind speed, pipeline operating temperature. And the failure time model of chemical pipe gallery in the jet fire accident was constructed by dimensionless analysis and processing, which realized the quantitative calculation of failure time and provided a theoretical reference for preventing the failure of pipe gallery pipeline caused by jet fire.

Space numerical simulation of full oxygen combustion flame in glass melting furnace based on flue gas preheating

This study explores the innovative design of a glass-melting furnace, which incorporates a combustion lance and a flue on one side. The research validates the simulation that the high-temperature flue gas, produced by the pure oxygen combustion of natural gas, can utilize residual heat in a multi-stage process to preheat and burn the fuel natural gas via a jet heat pump. This process significantly reduces energy consumption, thereby decreasing construction costs and energy usage in industries related to glass furnace production. The research focuses on the flame space of a 600 t/day natural gas flat glass all-oxygen combustion furnace. Utilizing numerical calculation methods, a three-dimensional mathematical model of the all-oxygen lance flame space was developed. Numerical simulation methods were employed to examine the temperature field, airflow field, and the distribution pattern of the liquid flow field within the glass space in the furnace. The simulation results reveal that incorporating high-temperature flue gas into the fuel can markedly enhance the performance of the spray gun flame. Additionally, choosing spray gun outlet designs of varying widths can tailor the flame width to better meet the requirements of the glass melting system, thereby significantly improving the waste heat utilization rate of the flue gas. This offers more alternatives for the direct utilization of flue gas. When compared to traditional oxy-combustion glass melting furnaces, this innovative design substantially reduces construction costs and is more energy-efficient and environmentally friendly.

Numerical study on thermal hydraulics characteristics during steam submerged jet condensation from rectangular CD nozzle

The phenomenon of steam-water direct contact condensation (DCC) has significant importance in nuclear industry due to its rapid and efficient heat and mass transfer rates. In this numerical study, three dimensional simulations of steam submerged jet condensation from rectangular converging-diverging (CD) nozzle into subcooled water has been investigated. The steam condensation phenomenon has been captured from thermal phase change model as user defined function (UDF) in ANSYS Fluent software. The computational results were validated using experimental results as well as previously published numerical study. The effects of steam pressure and water temperature on distributions of various thermal hydraulics parameters including velocity, pressure, temperature, heat and mass transfer characteristics have been studied. The steam pressure and water temperature have been kept in the range of 200–500 kPa and 20–60 °C, respectively. The maximum value of jet velocity and heat transfer coefficient have been found as 803 m/s and 2.73 MW/m2K respectively. The peak velocity value increases with steam pressure. After maximum expansion point, the velocity profile curve is shifted downstream with steam pressure. The axial pressure distribution first decreases and then increases from expansion-compression waves. The axial temperature profile followed the similar trend as observed for axial steam pressure distribution. At constant steam pressure, the maximum rate of heat transfer coefficient decreases with the increase in water temperature. The maximum mass transfer rate under prescribed flow conditions has been found as 2789.8 kg/m3s. The peak mass transfer rate decreases with steam pressure and water temperature. The peak rates for heat and mass transfer have been observed in the nearby vicinity of nozzle exit location.

Study of active and passive methods to control flow, heat and mass transfer in jets

The numerical experiment results for three cases with active and passive methods to control jet flows are presented. In problem 1, transverse vibrations of the inlet boundary and longitudinal oscillations of the inlet velocity are applied to obtain the flow splitting and to enhance mixing. In problem 2, perturbations at the lateral boundaries are introduced for the same purpose. In problem 3, a grid is introduced at the entrance into the impinging jet to enhance heat transfer on the heated wall with which the jet collides. The obtained effects are discussed in comparison with the measurement data, and prospects for further research are indicated.

Numerical Study and Structural Optimization of Impinging Jet Heat Transfer Performance of Floatation Nozzle

A floatation nozzle can effectively transfer heat and dry without touching the substrate, and serves as a vital component for heat transfer to the substrate. Enhancing the heat transfer performance, and reducing its heat transfer unevenness to the substrate play an important role in improving product quality and reducing thermal stress. In this work, the effects of key structural parameters of the floatation nozzle on the heat transfer mechanism are systematically investigated by means of a numerical simulation of computational fluid dynamics. The findings demonstrate that the secondary vortex structure induced by the floatation nozzle with effusion holes increases heat transfer performance by 254.3% compared with the nozzle without effusion holes. The turbulent kinetic energy and temperature distribution between the jet and the target surface are affected by the jet angle and slit width respectively, which change the heat transfer performance of the float nozzle in different degrees. Furthermore, to improve the comprehensive heat transfer performance of the floatation nozzle structure, taking into account the average heat transfer capability and heat transfer uniformity, the floatation nozzle’s design is optimized by the application of the response surface method. The comprehensive heat transfer performance is increased by 26.48% with the optimized design parameters. Our work is of practical value for the design of floatation nozzles with high heat transfer performance to improve product quality in industrial systems.

Study on Three-dimensional Impinging Jet Flow and Heat Transfer of Dryer for Gravure Printing and Coating Machine

Drying system is the largest energy consumption of equipment parts of solvent-based printing and coating equipment. How to optimize to design the equipment is of the highlights of this research field. According to this demand, a mathematical model of fluid flow of the drying oven is developed. By building a testing platform and using RNG k-epsilon two-equation turbulence model in Fluent Software to analyze three-dimensional impinging jet flow and heat transfer process of hot air drying in conventional drying oven. The results show that the separation of some boundary layers and undesirable swirls, which result in the non-uniform velocity field and temperature field of hot air from nozzles and no promotion to the drying effect, because of unreasonable design of the drying oven.

Exact solutions for permeable wall laminar jet with velocity slip: Momentum and thermal profiles

This study presents exact analytical solutions for the two-dimensional laminar jet flow and heat transport of Casson fluid under velocity slip condition. The laminar wall jet of classical Glauert type is extended to the Casson fluid model considering the flow to be along a flat permeable surface. The roles of velocity slip, wall temperature, and mass transfer on the fluid flow and rate of thermal exchange are investigated. The outcomes for momentum and thermal fields, skin friction and heat transfer gradient are obtained analytically with numerical solutions of the suction-far stream velocity expression. It is noted that mass transfer and velocity slip have significant effects on the flow of fluid and heat transfer. Thermal profile varies significantly for increasing Prandtl values with small suction.