Counterflow Configuration Research Articles

Numerical study on the influence of flow direction and fluid type on heat transfer and pressure drop in a two-tube spiral heat exchanger with innovative conical turbulators

In this research, numerical analysis will be conducted on the thermal and hydraulic performance of a spiral two-tube heat exchanger. The study employs an innovative turbulator design with curved outer blades and a semi-conical section with two openings. Numerical simulations were conducted using the commercialized Finite Volume Method (FVM) program, ANSYS FLUENT 18.2. The finite volume method was employed to analyze thermal and fluid equations and parameters. A comparison was made between two distinct flow directions, namely positive and negative currents, utilizing a comprehensive range of three working fluids. The numerical findings indicate that employing counter-current flow with a Water/MOS2−Fe3O4 fluid nanohybrid results in superior endothermic performance. These results indicate that a maximum thermal efficiency improvement of roughly 17.54 % and 8 % can be achieved by using the counterflow configuration with the nanohybrid fluid Water/MOS2−Fe3O4 at a mass flow rate of m˙ = 0.008 kg/s, in comparison to alternative models. Furthermore, the results showed that the Water/MOS2−Fe3O4 fluid nanohybrid performs better than the Ag-HEG/Water nanohybrid fluid when used in counterflow and parallel flow configurations, starting at a Reynolds number of 600. With a counterflow configuration in the two tubes, the Water/MOS2−Fe3O4fluid nanohybrid achieves the highest thermal performance coefficient in the two-tube heat exchanger.

Effect of Chevron Angles on the Thermal Performance of Corrugated Plate Heat Exchanger (CPHEs)

Corrugated plate heat exchangers (CPHEs) possess a greater surface area for heat transfer and a heightened level of turbulence as a result of the corrugations. This study presents an experimental analysis comparing the thermal performance of a flat plate heat exchanger and a corrugated plate heat exchanger (CPHE) with varied corrugation angles. The analysis was conducted using water as the test fluid. The comprehensive testing has been carried out on CPHEs with different chevron angles (β) of 30°/30°, 45°/45°, and 60°/60°. Experimental data is collected under steady- state conditions, with a single phase (water-water) system, covering Reynolds numbers (Re) ranging from 900 to 1250. The test fluid's mass flow rate, ranging from 0.05 to 0.16 kilograms per second, is measured together with the accompanying steady state temperatures. Each plate was equipped with four thermocouple sensors to detect the bulk temperature of the cold and hot fluids at the intake and output. The performance was evaluated using both parallel and counterflow configurations. Experimental observations are utilized to estimate the temperature difference (∆T) between the inlet and output streams, the logarithmic mean temperature difference (LMTD), and the exergy (E). The measured ∆T and E values for corrugation angles (30°, 45°, and 60°) of CPHE were compared to those of flat plate heat exchangers. For corrugation angles of 30°, 45°, and 60°, the values of ∆T and E (efficiency) increase as the mass flow rate of the fluid increases. An increase in the corrugation angle leads to an increase in turbulence in the flow, which in turn enhances heat transmission. In addition, the thermal effectiveness (ε) was calculated using the NTU method and compared for all of the plates. Key Words: Corrugated plate heat exchanger, Thermal performance, Chevron angle, heat transfer coefficient.

Operando X-ray radiography of liquid water distribution in 100 mm polymer electrolyte fuel cell channels

Preventing the accumulation of water in the gas diffusion layer (GDL) proves effective in enhancing the performance of polymer electrolyte fuel cells (PEFCs). To understand the water transport phenomena in GDLs and channels of PEFCs, cell hardware for operando synchrotron X-ray radiography was developed with a 100 mm channel length, facilitating the separate quantification of liquid water in the cathode and anode GDLs. The presence of liquid water in the cathode and anode GDLs was confirmed during operation with a supply of dry gas in a counter-flow configuration. Furthermore, the amount of liquid water in the cathode and anode GDLs increased toward the cathode inlet, while the amount of water in the cathode and anode channel regions increased toward each outlet. The liquid water distribution in the GDLs along the channel direction can be attributed to water transport from cathode to anode (back-diffusion), decreasing toward the anode outlet. Therefore, conducting radiography experiments aligned parallel to the GDLs and perpendicular to the channel could provide valuable insights for a more comprehensive understanding of water transport in cells.

Heat recovery of multistage circulated permeate gap membrane distillation for energy-efficient water production

Addressing the challenges of energy efficiency in the desalination industry, this study proposed an energy-efficient water desalination system, focusing on the novel innovative approach of gap water circulation in permeate gap membrane distillation (PGMD) systems. A counterflow multistage is considered and analyzed using combined heat-mass transfer modeling. In comparison to conventional PGMD configurations, the introduction of gap water circulation has visibly emerged to increase water productivity while decreasing energy consumption. This research systematically explored the impact of this innovation, employing a comprehensive methodology that includes modifying flow chamber configurations and analyzing operational parameters. The study was performed at different seawater intake flow rates, intake temperature, feed injection temperature, and different number of stages. It was found that increasing the number of stages in the counterflow configuration presented better energy efficiency and higher feed recovery ratios at different conditions. A remarkable reduction in Specific Thermal Energy Consumption (STEC) to 92 kWh/m3, coupled with a notable increase in flux values reaching 28 kg/m2.hr for the alternative modules due to the intensified turbulence and mixing. The study demonstrated the potential of gap water circulation in significantly enhancing the energy efficiency and productivity of MD systems compared to conventional PGMD systems.

Investigations on adsorption – Evaporation characteristics and cycle time effects of integrated desiccant coated M-cycle cooler

This paper looks into the possibility of undertaking simultaneous dehumidification and evaporative cooling in a single heat exchanger, through the conceptualisation of a desiccant-coated M-cycle cooler. The cooler utilises waste heat as a low-grade thermal energy input to regenerate the silica gel desiccant. The interplay between the adsorption and evaporation phenomena has been studied for the crossflow and counterflow configurations of the cooler. Partial differential equations have been formulated for both heat and mass transfer across each of the control volumes. Results obtained from the transient analysis reveal three distinctive temperature regimes of the counterflow configuration and two regimes for the crossflow configuration. It is also observed that the evaporative cooling effects are more predominant in the counterflow setup. Cycle time analysis reveals that the counterflow configuration has greater operational times and lower downtimes as compared to the crossflow configuration. At 82.5 °C, the operational time of counterflow configuration is observed to be double the downtime, thereby doubling its cooling capacity. Thus, the study showcases the potential of utilising low-grade waste heat to develop an integrated adsorption-evaporation cooler for air-conditioning applications.

CFD modeling of a tubular three-pass solar air heater with phase change material: Investigating heat transfer enhancement and energy efficiency

The present study introduces a novel solar air heater design featuring a tubular surface with three passes arranged in a counter-flow configuration, aimed at improving compactness and enhancing heat transfer characteristics. The research involved the development and validation of a computational fluid dynamics (CFD) model, which was utilized to accurately depict the distribution of solar flux through the system components by comparing the model results with experimental data. The study involved conducting a comparative analysis between the tubular three-pass solar air heater with phase change material (TTPSAH_PCM) and conventional flat plate solar air heaters (FPSAH) to ascertain the superior design characteristics. A thorough investigation was carried out, encompassing evaluations of energy, exergy, outlet temperature, convection heat transfer coefficient, and useful heat gain. The findings indicate that the TTPSAH_PCM outperforms alternative geometries. For instance, when operating at a mass flow rate of 0.006 kg/s, the TTPSAH_PCM demonstrates substantial enhancements in several key metrics, including an 11.4 % increase in the system's daily thermal efficiency, a 17 % improvement in exergy efficiency, and a 4 % rise in outlet air temperature. The convection heat transfer coefficient of TTPSAH_PCM exhibited a 155.6 % increase compared to the FPSAH type. Moreover, the system being investigated demonstrates an 8.5 % rise in useful heat gain. The results underscore the enhanced efficacy of the TTPSAH_PCM system in contrast to alternative solar air heater arrangements, and its applicability for both drying operations and indoor heating applications.

Dielectric barrier discharge actuators: Momentum injection into co-flow and counter-flow freestream

In a quiescent environment, dielectric barrier discharge (DBD) plasma actuators generate a wall jet through the interaction of ionized and neutral molecules in an electric field. In external flow, the coupling between electrohydrodynamic (EHD), turbulence, inertial, and viscous effects in the flow boundary layer is more complex and requires additional investigation. We experimentally study momentum injection by DBD actuators into the free stream flow with Re = 35,000 and 75,000 in co-flow and counter-flow scenarios over a range of VAC = 12 kV–19.5 kV peak-to-peak at a frequency of 2 kHz. In co-flow, the momentum injection leads to boundary layer thinning and fluid entrainment from the freestream into the DBD forcing region, while in the counter-flow configuration, flow separation can occur. A separation bubble is observed at Re = 35,000 for the tested condition. The momentum difference in the counter-flow configuration is six times greater than the EHD jet momentum in a quiescent environment. Both co-flow and counter-flow momentum injections show diminishing effects with higher Re. We show that the resulting flow pattern is not a superposition of the EHD jet and the free stream but is determined by the coupling and competition of inertial, viscous, and Coulombic effects between the EHD-driven forcing and the external flow. The proposed non-dimensional momentum ratio (M*) of EHD jet momentum to momentum in the external flow boundary layer can be used to predict the onset of separation; however, additional experimental and numerical studies are required to generalize this concept to other flow scenarios. The velocity profiles and momentum measurements presented here can be used to validate numerical models and inform the design of DBD actuators for active flow control.

Heat Transfer of an Integrated Counterflow Ceramic Heat Exchanger

The ceramic-based heat exchanger is one of the leading contenders for high-efficiency concentrating solar power plants using a molten salt heat transfer fluid and a supercritical carbon dioxide Brayton power cycle operating at temperatures above 700 °C due to the excellent resistance of ceramics to corrosion, oxidation, erosion, creep, and fouling. In the present study, the thermal performance of a ceramic silicon carbide prototype heat exchanger, with semi-elliptical heat transfer channels, integrated header channels, and a counterflow configuration fabricated by using binder jetting additive manufacturing, was experimentally investigated. Experimental heat transfer tests of the prototype were conducted at high temperatures and under various test fluid flow rates and inlet temperatures. The experimental heat transfer rates compared favorably with simulation predictions.

Experimental study on the structure and propagation of the stretched hydrogen-rich turbulent premixed flames in the thin-reaction-zone regime

An experimental study on the structure and propagation of the stretched hydrogen-rich premixed turbulent flames in the thin-reaction-zone regime was conducted. A set of optical diagnostic techniques including the 2D/3D-particle image velocimetry, hot wire anemometer, intensified charge-coupled device (ICCD), and laser tomography visualization (LTV) methods were adopted to quantitatively measure the velocity field, turbulent intensity, and thickness of the stretched flame of the under various conditions. A new method to improve the measurements of the flame preheat zone thickness was developed by using the low-temperature available LTV method to identify the onset of the preheat zone and using the peak location of CH* to identify the reaction zone. The results showed that the flow field was in the inertial turbulent subrange and the turbulent flames obtained in the present counterflow configuration were categorized into the thin reaction zone regime in Borghi/Peters turbulent flame diagram. The experimental results showed that the flame structure of a turbulent H2-rich premixed flame was synergistically influenced by the hydrodynamic stretch, turbulence, and preferential diffusion. New evidence of the phenomenon accumulated that large-scale vortices wrinkled the surface of the flame while small-scale vortices penetrated into the flame preheat zone. Scaling laws for the turbulent flame propagation speed, apparent turbulent thermal conductivity, and were given with the consideration of the synergetic effect of hydrodynamic stretch, turbulence, and preferential diffusion. The scaling of the turbulent flame preheat zone thickness turned out to be sensitive, while those of the turbulent flame propagation speed and apparent turbulent thermal conductivity were insensitive, to the bulk flow strain rate.

Comparative study on energy-saving performance of single- and two-stage evaporative-cooling condenser systems

The mechanical vapor compression system (MVC) coupled with the evaporative-cooling condenser is an effective method to improve its high temperature adaptability. However, this hybrid systems typically use an evaporative cooler as a single-stage pre-cooling unit of condenser and is mainly designed for hot–dry climates. In this paper, based on conventional evaporative-cooling condenser configuration, three two-stage evaporative-cooling condenser systems (MVC-TSEC(A), MVC-TSEC(B) and MVC-TSEC(C)) were proposed. The MVC-TSEC(A) consists of one condenser and an indirect/direct evaporative-cooler. The MVC-TSEC(B) and MVC-TSEC(C) have two condensers (in series), which are coupled with a two-stage evaporative cooling system. The refrigerant in the MVC-TSEC(C) forms a counter-flow configuration with the outdoor air, while it is concurrent flow in MVC-TSEC(B). Subsequently, the effects of the ambient parameters and outdoor air flowrate on the three two-stage hybrid systems were analyzed comparatively based on detailed numerical models. Finally, the application potential of these hybrid systems was evaluated comprehensively in hot–humid climates. The results showed that the seasonal COP and energy-saving rate for the MVC-TSEC(C) are reached by 4.2–4.7 and 12.7–21.1 %, respectively. Moreover, the static equipment payback period of the three two-stage hybrid systems are 3.8, 3.3 and 3.0 years, respectively.

Experimental and numerical study of PMMA combustion in counterflow configuration

PMMA burning in counterflow has been studied experimentally and numerically. Thermal and chemical structure including concentrations of main species, i.e. ММА, O2, N2, CO2, CO and H2O, as well as an integral parameter (mass loss rate of a sample) were measured. The mathematical model is set in a coupled gas–solid formulation to resolve heat and mass transfer between gas-phase flame and thermally degrading solid combustible. Gas-phase combustion was modeled in the open-source Cantera software in a one-dimensional counterflow flame configuration. Governing equations of solid fuel was solved in an in-house code in two-dimensional formulation. Two chemical mechanisms of MMA oxidation (skeletal and detailed) were employed to resolve gas-phase combustion. Two sets of parameters of PMMA pyrolysis reaction were investigated. Gas phase temperature distribution was experimentally shown to be almost uniform in the direction parallel to the sample's burning surface in the area up to 4 mm from the sample's axis. The best agreement for macroscopic parameter, mass loss rate, was obtained by employing Lengelle pyrolysis kinetics, which is within 10 %. Total heat flux from gas-phase flame to the solid sample is about 23 kW/m2. Both gas-phase combustion mechanisms overpredict maximal temperature in flame by about 200 °C.

Numerical investigation of latent heat storage unit with dual helical tube

The application of latent heat storage systems in waste heat recovery has become a promising solution for energy storage. This study proposes a configuration using dual helical tube, specifically designed for industrial waste heat recovery applications. This study investigates the thermal performance of a horizontal dual helical energy storage unit under natural gravity. It considers various parameters, including helical diameter, the number of helical turns, and eccentricity. When an eccentricity of 5 mm is used, the overall energy storage time is reduced by 30.86 %, and the energy storage density is at its highest (3.11 × 10−4 kW/cm2). Adjusting the outer helical tube to 9 turns produces the highest energy storage efficiency (0.9252). Additionally, the heat transfer characteristics during simultaneous energy storage and release processes have been studied. The results indicate that as the temperature difference or velocity ratio between the heat transfer fluids increases, the liquid fraction at steady state decreases. This decrease in liquid fraction requires a longer time to reach the steady state. When the outer helical tube is filled with the hot fluid and counter flow configuration is adopted, it achieves higher levels of absorption and release power at steady state. The influence of velocity on absorption and release power is more significant than that of temperature difference.

Improved flow boiling performance and temperature uniformity in counter-flow interconnected microchannel heat sink

Microchannel flow boiling is one of the most effective solutions to the problem of heat dissipation caused by high power electronic devices. However, inherent drawbacks such as low critical heat flux due to dry-out of the liquid film near the channel exit, relatively high two-phase flow pressure drop, severe temperature nonuniformity and boiling instability at high heat flux still prevent widespread commercial application. In this study, based on the idea of uniform vapor quality along the flow direction, a copper Counter-Flow Interconnected Microchannel (CFIM) was proposed to address the main issues currently faced by two-phase microchannel heat sink. A comprehensive comparative study of CFIM, Co-Current Microchannel (CCM) and Counter-Flow Microchannel (CFM) was carried out from the aspects of boiling stability, boiling heat transfer characteristics and temperature uniformity. Three slots of 0.2 mm, 0.5 mm and 0.8 mm in width, denoted as IM0.2, IM0.5 and IM0.8, respectively, are considered. Experiments are conducted at mass fluxes of 291–618 kg/m2·s, saturation temperature of 40 °C and effective heat fluxes of 3–293 W/cm2, using the dielectric fluid R1233zd(E) as the working fluid. The results show that the CHF and average two-phase heat transfer coefficient (HTC) of CFIM are increased by 40.6 ∼ 100.5 % and 70.4–83.6 %, respectively, while the two-phase pressure drop is decreased by 17–32.6 % compared to CCM. The design of the counter-flow interconnected structure can manipulate the void fraction and two-phase flow pattern, resulting in a near uniform void fraction and flow pattern. The structure of these connecting slots promotes nucleate boiling and fluid mixing of neighboring channels. More importantly, the flow boiling instability and temperature non-uniformity in CFIM are well suppressed. In addition, IM0.2 has higher heat dissipation than IM0.5 and IM0.8 due to enhanced bubble nucleation, reducing superheat requirements and periodic rewetting to the center of the channel. The results of this research reveal that utilizing a microchannel heat sink with a counter-flow interconnected configuration can effectively improve heat transfer performance during flow boiling. Furthermore, it helps to mitigate issues such as flow boiling instability and temperature non-uniformity.

A NUMERICAL STUDY ON THE REDUCTION OF GREENHOUSE GASEOUS COMPONENT (CO2) DUE TO THE ADDITION OF H2 IN THE FUEL STREAM OF THE COUNTERFLOW CH4/AIR DIFFUSION FLAME

In this study, a series of 1-D and steady-state numerical simulations have been performed for the prediction of the effect of the addition of H2 on the characteristics of a non-sooting counterflow CH4/Air diffusion flame using detailed chemical reaction model, which is composed of 325 elementary chemical reactions and 53 chemical species. Under the steady-state assumption, a set of one-dimensional transport equations of mass, momentum, species, and energy along with the equation of state has been solved numerically at the atmospheric conditions over the counterflow configuration by exploiting an efficient numerical code, OPPDIF (a Fortran Program for Computing Opposed-Flow Diffusion Flames). The grid adaption technique has been used to achieve better convergence as well as to ensure the maximum accuracy of the simulated results. It is found that the flame temperature is increased due to the addition of H2 with CH4, which is injected into the fuel stream. The elevation in the temperature is caused by the augmentation of the integrated heat release rate of the elementary reactions supported by the active radicals (H, O, and OH), which are generated by the higher reactivity of H2. Besides, it is found that the mole fractions of H2O are increased as the percentage of H2 in the loading fuel (CH4) is increased and also, it is identified that the chain propagating reaction, OH + H2 => H2O + H is dominating one which produces highest amount of H2O. Furthermore, it is noticed that the indirect greenhouse gas or precursor, CO is reduced when H2 is added to CH4. Consequently, the mole fraction of the principle greenhouse gas, CO2 is decreased significantly when the fuel, CH4 percentage is modified by the higher percentage of H2. The sensitivity analysis of elementary reactions reveals the fact that the chemical reaction: OH + CO => H + CO2 is a dominating reaction in producing a lower amount of CO2 when the volume fraction of H2 is increased in the fuel (CH4) stream. In the presence of 75 % H2 in CH4, the pressure-dependent reaction, O + CO (+M) => CO2 (+M) appears as another chemical route that also generates greenhouse gas, CO2 but its contribution is negligibly small.

The Effects of Stack Configurations on the Thermal Management Capabilities of Solid Oxide Electrolysis Cells

In this study, we analyze the impacts of various stack configurations of a solid oxide electrolysis cell (SOEC) that includes U-type and Z-type stack structures as well as co-flow and counter-flow configurations. The primary focus of this study is to analyze the impact of these SOEC stack configurations on the temperature distribution within the stack and the temperature variations of key components. Furthermore, by predicting the thermal stress and thermal deformation of individual SOEC components, the study can provide design guidelines for enhancing the durability of the SOEC stack. Among various SOEC stack configurations, the counter-flow design outperformed others in temperature uniformity and component temperature variation. The Z-type stack structure slightly surpassed the U-type in flow uniformity, while both had a minimal influence on thermal management. Besides conventional flow-field configurations, such as the parallel flow field, we introduce a metal-foam-based flow-field design and analyze the effects of using metal foam to ensure flow uniformity within the stack and achieve temperature uniformity. The metal foam design has a lower average temperature (2–5 °C) and ∆T (4–7 °C) compared to the parallel flow field in each cell, but this improvement is accompanied by a substantial pressure-drop: 2359.3 Pa for vapor flow (11.7 times higher) and 4409.0 Pa for air flow (4.6 times higher). Additionally, structural analysis was performed using CFD temperature data. The co-flow configuration induced higher thermal stress at the front of the stack, whereas the counter-flow configuration mitigated thermal stress in the front cells. The metal foam structure consistently demonstrated a reduction in thermal stress across all cells by about 1 MPa, highlighting its potential to alleviate thermal stress in SOEC stacks. This study presents a novel CFD analysis approach for a 10-cell SOEC stack, enabling the development of an optimized stack design with improved heat and flow distribution. The integrated CFD–FEM analysis provides reliable thermal stress data that elucidates the correlation between temperature and stress distributions within the stack.

Investigation of the interaction mechanism between power and heat of a reversible solid oxide cell during a mode switching cycle

Chemical energy and electrical power can be converted bi-directionally with unrivaled efficiency by the mode switching of reversible solid oxide cell (RSOC) stacks. Mode switching can lead to temperature fluctuations in RSOC stacks, which will significantly affect the mechanical stability of stacks. To investigate the transient performance of the RSOC, a 1D dynamic model of a single repeating unit (SRU) is established and validated against experimental data. The interaction mechanisms between the thermodynamic and electrochemical parameters of the RSOC during transients are elucidated elaborately. Two indicators, i.e., temperature variation rate and temperature gradient, are used for the quantification analysis. The effects of current density, hydrogen fraction, and inlet gas temperatures on the temperature distribution along the SRU are investigated. In the simulation scenarios, the maximal temperature variation rate can achieve 1.49 and 1.495 K min−1 in co- and counter-flow configuration, and the maximal temperature gradient can achieve 5.6 and 4.6 K cm−1, respectively. The hydrogen fraction has a significant influence on the temperature. Increasing the hydrogen fraction from 0.4 to 0.7, the maximal temperature variation rate decreases by 12.8 % and 12.1 % in the co- and counter-flow SRU, and the maximal temperature gradient reduces by 10.6 % and 14.2 %, respectively.

Passive Mixing and Convective Heat Transfer Enhancement for Nanofluid Flow across Corrugated Base Microchannels

Vortex generators and pin fins are conventionally used to deliver fluid mixing and improved convective heat transfer. The increased pressure loss following a fractional increase in heat transfer, as well as the complex manufacturing design, leave room for improvement. The present work proposes a novel diverging–converging base corrugation model coupled with vortex generation using simple geometrical modifications across rectangular microchannels to ensure a superior performance. The Nusselt number, friction factor, and flow phenomenon were numerically studied across a Reynolds number range of 50–1000. The optimum cross-section of the microchannel-generating vortices was determined after thorough study, and base corrugation was further added to improve heat transfer. For the vortex–corrugation modeling, the heat transfer enhancement was verified in two optimized cases: (1) curved corrugated model, (2) interacting corrugated model. In the first case, an optimized curve generating Dean vortices was coupled with base corrugation. An overall increase in the Nusselt number of up to 32.69% and the thermal performance of “1.285 TPF” were observed at a high Reynolds number. The interacting channels with connecting bridges of varying width were found to generate vortices in the counter-flow configuration. The thermal performance of “1.25 TPF” was almost identical to the curved corrugated model; however, a major decrease in pressure, with a loss of 26.88%, was observed for this configuration.

Numerical Evaluation of Heat Transfer and Conversion Efficiency by Tube Design and Flow Configuration for a Compact Steam-Methane Reformer

In the chemical industry, hydrogen (H2) production through steam-methane reforming is a well-established process. With the growing demand for H-fueled vehicles and charging stations, there is a need for compact reformers with efficient heat transfer capabilities. In this study, computational fluid dynamics simulations were performed to evaluate the methane (CH4) conversion and heat transfer efficiency of various reformer designs. These designs include single, double, and triple tubes, each with parallel- and counter-flow configurations between the reformate feed and heat source. The findings revealed substantial disparities in methane conversion between the tube designs and flow configurations. Notably, the triple-tube design outperforms single and double tubes, exhibiting higher methane conversion and improved heat transfer efficiency. This superior performance is attributed to the larger wall area facing the heat source and additional heat recovery from the reformate flowing in the inner annulus. This led to the highest temperature at the catalyst exit among the cases, increasing methane conversion, and the lowest reformate temperature at the reformer tube exit, which is also beneficial for the subsequent water–gas shift reaction process. Installing external fins on the reformer tube provided a more effective enhancement of heat transfer than using internal fins in the catalyst section. Regardless of the tube design employed, the counter-flow configuration consistently enhanced the heat transfer efficiency, resulting in 4.6–11.9% higher methane conversion than the parallel-flow configuration. Consequently, the triple-tube design with the counter-flow configuration achieved the highest methane conversion, offering flexibility in the reformer design, including the potential for lower heat input and a reduced catalyst volume.

Forced ignition of premixed cool and hot DME/air flames in a laminar counterflow

Recently, low-temperature chemistry (LTC) and LTC-induced cool flames have attracted extensive interest since they can substantially influence the ignition process and accelerate the subsequent hot flame propagation. However, it is unclear how the flow affects the forced ignition of a cool flame and its subsequent transition to a hot flame. In this study, we conduct transient 2D simulations for the forced ignition processes of premixed cool and hot flames in a laminar, axisymmetric, counterflow configuration with well-defined flow field. Different ignition energies and strain rates are used to ignite and stabilize cool and/or hot flames in a counterflow. The critical conditions for the ignition of cool and hot flames are identified, and the cool flame kernel quenching and its transition to hot flame are discussed. It is found that the ignition energy determines the highest temperature and thereby controls the thermal runaway process. The strain rate influences the flame kernel propagation and the transition from cool flame to hot flame. A large strain rate may quench the cool flame, while a small strain rate may lead to the transition from cool flame to hot flame due to the sufficiently long residence time. Therefore, cool flame ignition and stabilization can only be achieved within a certain range of strain rates. Furthermore, an ignition regime diagram in terms of ignition energy and strain rate is proposed, and four regimes are identified: (I) only cool flame, (II) only hot flame, (III) ignition failure, and (IV) transition from cool flame to hot flame (with appearance of double flame structure). These results provide new insights into how the flow affects the cool flame ignition and transition.

Analysis of optimized combined microchannel and heat pipes for electronics cooling

In this work, innovative heat sink designs consisting of Microchannel Heat Sinks (MCHS) and Flat Plate Micro Heat Pipes (FPM-HP) are investigated to improve thermal management in electronic components. The three-dimensional finite volume method is used to analyze the fluid flow and heat transfer in the heat sinks. The validity of the numerical models is confirmed by comparison with pertinent experimental data. The thermal performances of the new heat sink designs are compared to that of double-layer microchannel heat sinks with parallel flow (DLPF MCHS) and counter flow (DLCF MCHS) configurations, as two of the more efficient heat sink designs in the industry. The investigated configuration of Silicon-Water Counter Dual MCHS with rectangular cross-section improves the temperature uniformity by 43–50 % and 26–31 % compared to DLPF and DLCF MCHSs, respectively, and the triangular cross-section configuration improves it by 38–47 % and 21–26 %, respectively. The thermal resistance of heat sinks is reduced by 10–15 % compared to DLPF MCHS and by 2–8 % compared to DLCF MCHS. Additionally, innovative hybrid heat sink designs combined with MCHS and FPM-HP are proposed to optimize hotspot management in electronics cooling applications that involve local high heat generation. The hybrid DLPF MCHS & FPM-HP reduces the maximum temperature and surface temperature gradient of the heat sink by 13–23 % and 20–49 %, while these are reduced by 16–40 % and 27–64 % for the hybrid DLCF MCHS & FPM-HP compared with the corresponding copper base (no heat pipe) designs. The hybrid Counter Dual Rectangular MCHS & FPM-HP reduces the maximum temperature and surface temperature gradient of the heat sink by 15–24 % and 29–65 % compared with the DLCF MCHS with copper base (no heat pipe) design, while these are reduced by 10–26 % and 24–63 % for the Counter Dual Triangular MCHS, respectively.