Heat Transfer Irreversibility Research Articles

Thermal performance analysis of a novel letter-type fin liquid cooling plate based on the field synergy principle and the second law of thermodynamics

In order to expand more forms of liquid-cooled plate fin construction, based on the traditional geometric fins and letter-type fins, a new idea of constructing a new type of letter-type fins by the geometric expansion method is proposed, and then a fin structure design with better heat transfer performance than the traditional geometric fins is explored. Firstly, the validity of the CFD model and method is verified by heat dissipation experiments. On this basis, the flow behavior and heat transfer performance of 6 letter-type fin structures at different Reynolds numbers are numerically investigated, and the synergism and thermal irreversibility of the flow and temperature fields are discussed and analyzed. According to the findings, the field synergy of the new letter-shaped fins is more significant and effectively reduced the irreversibility of heat transfer. In addition, the combined thermal performance analysis shows that the combined heat performance of C-type fins is better when the Reynolds number range is between 49.85–199.4 (HTPF = 1.098–1.57), and the combined thermal performance of T-type fins is better when the Reynolds number range is 249.25–398.8 (HTPF = 1.03–1.07). Secondly, in order to improve the liquid-cooled plate’s heat performance the paper discussed and optimizes the combination mode of C- and T-type letter fins. To further improve the liquid-cooled plate’s thermal performance, on the basis of the optimal combination of fins, we discuss the effect of fin break distance on the liquid-cooled plate’s thermal performance. Finally, secondary fins are introduced to further optimize the liquid-cooled plate’s thermal performance. It is found that a reasonable setting of secondary fins can effectively reduce the pressure drop of the liquid cooling plate and improve the comprehensive heat dissipation performance (18.71 %∼32.09 %).

Unlocking Heat Transfer Potential in Multi-Sided Porous Geometries: A Study on Magnetothermal Effects and Entropy Generation

This study investigates heat transfer efficiency and irreversibility generation in multi-sided porous thermal systems filled with a Cu-Al2O3-water hybrid nanofluid. The research examines convective heat transfer across various polygonal geometries, ranging from four-sided to infinitely-sided (circular) structures, aiming to maximize thermal system performance intended for scientific and industrial applications. For a standardized comparison, the fluid volume and active heating and cooling lengths are kept constant for all the geometries considered. The analysis includes varying flow-controlling variables like the modified Rayleigh number (Ram), Hartmann number (Ha), and Darcy number (Da) to evaluate how they affect heat transfer efficiency and entropy generation, including total, magnetic, and viscous entropy. The numerical simulations, conducted using the finite element approach, explore Ram values ranging from 10 to 104, Ha from 0 to 70, and Da from 10-4 to 10-2. Streamline and isothermal contours analyze flow behavior, while heatline tools reveal thermal energy transport dynamics. Results show up to a 20% improvement in heat transfer efficiency in optimized configurations. Key findings indicate that hexagonal structures can be viable alternatives to circular tubes in space-constrained applications, offering comparable thermal performance. Interestingly, pentagonal cavities exhibit the highest irreversibility due to symmetry loss. The study offers insightful information for enhancing thermal system design in various industrial settings, particularly for applications requiring efficient heat transfer in limited spaces.

3D MHD convection and entropy production in a rectangular cavity with localized heating: A study on conductive fluid dynamics

This study aims to investigate the magnetoconvection of an electrically conducting fluid within a rectangular enclosure through a comprehensive three-dimensional computational analysis. The enclosure has a hemispherical block for bottom heating and incorporates two elliptical sources with differing temperatures along its vertical sidewalls. The primary focus is to optimize heat transfer and fluid flow characteristics within this thermal system by analyzing various control parameters. The simulations were carried out with the relevant parameters of the problem, including the Rayleigh number [Formula: see text], Hartmann number [Formula: see text], the inclination of the magnetic field [Formula: see text], and the radius of the hemispherical block [Formula: see text]. Three distinct scenarios represent different positions for the localized heat sources. Among these configurations, the Middle–Middle (MM) arrangement is the most favorable, with the specific value for the radius [Formula: see text] yet to be determined. The findings highlight the effective control of heat transfer rate and irreversibility effects by manipulating the parameters [Formula: see text], [Formula: see text] and [Formula: see text]. It is demonstrated that selecting [Formula: see text] and [Formula: see text] values of [Formula: see text] and 15 allows for optimal thermal system configuration. A correlation with two variables [Formula: see text] expressing the rate of heat transfer through the cavity was established and verified. The analysis of the helicity confirms the predicted optimal case.

Entropy analysis of mixed convective electro-magnetohydrodynamic couple-stress hybrid nanofluid flow with variable electrical conductivity in a porous channel

The paper presents a novel study to examine the irreversibility of quadratically mixed convective electro-magnetohydrodynamic (EMHD) flow of a couple-stress hybrid nanofluid (CSHNF) with variable properties in a vertical porous channel. The channel walls are exposed to an applied electric field effect and a uniform transverse magnetic field. The hybrid nanofluid considered is an ethylene glycol (C 2 H 6 O 2) base mixed with multi-walled carbon nanotubes (MWCNT) and silver (Ag) nanoparticles (NPs), assuming the base fluid and nanoparticles to be in a state of thermal equilibrium following the Tiwari-Das nanofluid model. The potential applications of the study can be in microfluidics to nanofluidics, particularly in developing cooling technologies, EMHD pumps, high-end microelectromechanical systems (MEMS), and lab-on-a-chip (LOC) devices used in bioengineering. A constant pressure gradient acting in the flow direction and the buoyancy effect under the quadratic Boussinesq approximation drive the flow. The governing momentum and energy equations are nondimensionalized using pertinent dimensionless parameters and solved by the semi-analytical homotopy analysis method (HAM). The entropy generation and the Bejan numbers are derived to examine the irreversibilities in the system. To investigate the rate of shear stresses and heat transfer, skin friction coefficients and Nusselt numbers on the channel walls are determined. The analysis emphasizes the influence of nanoparticle concentration and electromagnetic field on the flow dynamics, temperature distribution, and system irreversibilities in the presence of porous media. It reveals the enhancement of fluid velocity and temperature degradation for higher concentrations. In contrast, both reduce for higher magnetic and electrical strength. With the enhancement of electrical joule heating and quadratic convection, a higher entropy generation rate is attained with a low rate of heat transfer irreversibility. However, it reduces with higher nanoparticle concentration, electrical strength, porosity, and variable electrical conductivity parameters under the dominance of heat transfer irreversibility.

Multistage data center cooling system for temperature gradation and matching

A prototype of a multistage cooling system with a cooling capacity of 20 kW is assembled and studied. The traditional single-stage system is split into two sub-systems according to the temperature level, which reduces the temperature difference and irreversible heat transfer loss, and further improves the system's energy efficiency. The system mainly features three modes including free-air cooling, refrigeration-based cooling and waste heat recovery, which can achieve efficient cooling while capable of recovering the waste heat to the surrounding heat users. The efficiency of the proposed cooling and heat recovery system was assessed by measuring its energy reuse effectiveness (ERE). The energy savings and environmental benefits of the system were analyzed across five cities in China, each representing different climates. Results indicate that the system achieved an ERE of 0.91 in Beijing, 38.9 % lower than the national average of the city. Additionally, an economic analysis showed a payback period of less than two years for the proposed system across all climates considered, which is 30 % shorter than that of a single-stage cooling system.

Thermodynamic evaluation of metal foams with partial filling in a pipe

This study presents numerical findings on the flow and heat transfer irreversibility when metal foams are partially filled in a horizontal pipe. A heater is embedded in the pipe's circumference with a known heat input. Aluminum metal foam, characterized by a pore density of 10 and porosity of 0.95, is placed next to the inner wall of the pipe to enhance heat transfer. To determine the optimal thickness of the metal foam for thermodynamic performance enhancement, metal foams of five different thicknesses (10–80 mm) are examined under forced convection heat transfer conditions. The study integrates the Darcy Extended Forchheimer and local thermal nonequilibrium models to predict flow and heat transfer characteristics through the metal foams. Validation of the numerical methodology is conducted by comparing the results with experimental data available in the literature. A novel aspect of this investigation is the application of the second law of thermodynamics to analyze the thermodynamic performance of metal foams. Exergy and irreversibility analyses are used to evaluate the thermodynamic performance, revealing that a pipe filled with metal foams up to a thickness of 40 mm exhibits superior thermodynamic performance compared to other cases examined in the study.

Entropy transport for quasi-one-dimensional flow

The study of entropy in the context of quasi-one-dimensional flow is expanded in this work. Specifically, a new entropy transport equation is derived and integrated into a closed-form algebraic expression for entropy change. The derivation identifies the respective components of the entropy change and is valid for flows with an arbitrary combination of area change, heat transfer, and friction. The irreversibility is identified and found to be composed of frictional dissipation, irreversible heat transfer, and irreversible flow work. The irreversible flow work is a new term that results from the restriction to quasi-one-dimensional flows. The algebraic expression for each component of the entropy change is first validated using several canonical flows (i.e., isentropic, Fanno, Rayleigh, and a normal shock). A unique entropy production mechanism is identified for each of the entropy producing canonical flows (e.g., Fanno, Rayleigh, and a normal shock). Two additional cases, sudden expansion and contraction, are then considered and show that irreversible flow work is the sole entropy production mechanism. Finally, simultaneous friction and heat transfer are examined, and the overall entropy change is decomposed into the respective contributions from frictional dissipation and the heat transfer terms. In all cases, the net entropy change from the newly derived expressions agrees with known solutions to within numerical precision.

Thermally developing streaming potential-mediated pressure-driven flow of Phan–Thien–Tanner fluid in a microchannel

In this study, we have conducted a semi-analytical investigation into the streaming potential-mediated pressure-driven flow of hydrodynamically fully developed and thermally developing viscoelastic fluid in a parallel plate microchannel. We have utilized a simplified Phan–Thien–Tanner model to describe the rheology of the viscoelastic fluid. Our approach delved into the full Poisson–Boltzmann equation, deriving exact analytical solutions for the electrostatic potential distribution and velocity profile. Furthermore, we concurrently derived semi-analytical solutions for the temperature distribution and Nusselt number, accounting for the effects of heat generation from viscous dissipation and Joule heating in thermally developing flows. We have demonstrated that an increase in the degree of surface charge triggers the streaming potential field, while the volumetric flow rate escalates with the viscoelastic parameter εWik¯2. Moreover, we have observed that the magnitude of the dimensionless temperature decreases with increasing values of the effective Joule heating parameter Speff. Our analysis reveals that the streaming potential effect hampers fluid flow, resulting in an increase in the bulk fluid temperature and consequently reducing the heat transfer rate. We observe that the magnitude of the Nusselt number decreases with increasing Speff. The entropy generation analysis reveals that increasing the Peclet number amplifies flow and temperature gradients, leading to higher fluid irreversibility in microchannels. The Bejan number experiences a significant decrease across the channel, reaching its minimum at a specific axial location before stabilizing further downstream. We find that heat transfer irreversibility predominantly influences system irreversibility, except for the Brinkmann number Br = 0.01, where convective heat transfer dominates at the entrance region, transitioning to friction losses beyond the thermal entry zone.

Entropy transfer efficiency-effectiveness method for heat exchangers, part 1: Local entropy generation number and operation performance limits

Here, to resolve the open problem of the second-law efficiency evaluation for heat exchangers, an efficiency-effectiveness method is developed from pure convection theory, where the entropy transfer efficiency defined as the outlet temperature ratio of cold and hot fluids exchanging entropy and local entropy generation number can measure the global and local irreversibilities of heat transfer. The efficiency and effectiveness are related by the inlet temperature ratio τ and capacity rate ratio φ, and their maxima subject to the local entropy generation number are determined for different flow arrangements. When τ = φ2, the effectiveness maxima are 0.5/(1 + φ) and 1/(1 + φ) while the highest efficiencies are φ and unity for parallelflow and counterflow arrangements, respectively. The optimum operating points of counterflow exchangers, at which the equal outlet temperature τT1∞ (T1∞ represents the hot inlet temperature) of two fluids with a critical capacity rate ratio τ induces uniform irreversibility, the effectiveness limit of 1/(1+τ), and the efficiency limit of unity, are obtained. A linear temperature difference method is also developed based on the redefined convective and overall heat transfer coefficients, invariably yielding an effectiveness equal to the number of heat transfer units. This method has potentials to be extended to wet heat exchangers with phase-change flows.

Thermodynamic optimization of heat exchanger circuitry via genetic programming

In this study, an evolutionary thermodynamic technique was developed for the optimization of heat exchanger circuitry. The proposed technique is capable of handling the unrestrained implementation of genetic operators while ensuring circuitry feasibility and basic manufacturability. The optimization tool was used to clarify the optimal heat transfer features in relation to the characteristics of the refrigerant and to provide a thermodynamic interpretation of the optimization results to extract general design guidelines. Evaporator circuitry optimization was conducted under given cooling capacity, superheating degree, and boundary conditions representative of air-conditioning applications. The consistency between the minimum entropy generation and the maximum coefficient of performance (COP) was demonstrated under these settings. Accordingly, heat exchanger configurations that take maximum advantage of the thermodynamic benefits of each refrigerant are proposed by optimizing the distribution of friction and heat transfer irreversibility. Consequently, the evaporator outlet pressure increases, thus lowering the compression ratio and maximizing the COP. The developed optimization method maximizes the benefits of low-GWP alternative refrigerants and shows that zeotropic mixtures may exhibit performance analogous to that of R32 and higher than that of R410A by approaching a Lorenz cycle operation.

Entropy generation on the dynamics of volume fraction of nano-particles and coriolis force impacts on mixed convective nanofluid flow with significant magnetic effect

Recent advances in simulating entropy generation for dissipative cross-materials with a quartic auto catalyst are reported. Entropy generation can be used to generate entropy in any irreversible heat transfer process, which is important in thermal machines. This study examines the impact of entropy generation on the thermal transport and momentum of a continuous two-dimensional dusty fluid magnetohydrodynamic (MHD) migration across an inclined stretched sheet. We also conducted an entropy generation study to discuss the effects of viscous dissipation, the volume proportion of dust particles, and mixed convection. The two-phase models that address the fluid phase and the solid phase have been discussed. As part of the flow model’s innovation, the effect of raising particulate matter concentration on the dynamic behavior of the fluid is investigated. The described model transforms the prevailing partial differential equations (PDEs) for two phases into a nonlinearly associated, nondimensional set of equations by implementing appropriate similarity alterations. For graphical results, MATLAB programming incorporates the bvp4c mechanism. This study indicates how to examine the impact of appropriate factors on the dusty phase of fluid and the non-Newtonian fluid phase. Both the entropy generation ( E gen ) and the Bejan quantity (Be) are presented in relation to their associated dimensionless factors. Parametric research is used to evaluate the impact of different flow parameters on temperatures, entropy generation, and Bejan numbers. The heat transfer rate using several quadratic regression models, which provide more clarity in determining physical parameters crucial to engineering, is assessed. It is observed that the entropy ( E gen ) and Bejan number (Be) declines with the rising mass concentration ( β ν ) of dusty particles. Also demonstrated that for both cases, increasing the magnetic influence and dust volume fraction ( Φ D ) resulted in a decrease in velocity profiles at the same time as temperature increased.

Free convection and entropy generation inside porous cavities with irregular vertical walls nonuniformly heated from below

In the current study, numerical analyses of heat transfer through free convection and entropy generation characteristics inside three distinct enclosures filled with porous media have been analyzed. The study involves exploring three cavity configurations that differ in the shape of their vertical irregular walls: wavy, convex and concave. The cavities were subjected to sinusoidal heat from below, with the left vertical walls being maintained at a constant cold temperature and the other walls are adiabatic. The governing equations were solved using the finite-element method. The parameters considered are the Darcy and Rayleigh numbers, paying attention to the implication of irregular shapes of the sidewalls. At high Rayleigh numbers, the Darcy number becomes more significant in influencing the Nusselt number and a more noticeable effect is observed with a higher Darcy number. Entropy generation due to heat transfer irreversibility is the dominant contributor of irreversibility at the lowest value of Darcy number while for higher values of Darcy and Rayleigh numbers entropy generation due to fluid friction becomes dominant. Moreover, when Rayleigh reaches 106, the irreversibility due to fluid friction is more significant in the case of the cavity with undulations on the sidewalls as compared to the two other cases. In contrast, the entropy generation associated with heat transfer is significant in the case of a cavity with a convex wall compared to the other two cases.

Numerical investigation of the overall thermal and thermodynamic performance of a high concentration ratio parabolic trough solar collector with a novel modified twisted tape insert using supercritical CO2 as the working fluid

Supercritical CO2 (sCO2) is favoured as the next-generation working fluid for advanced high-temperature and high-concentration ratio parabolic trough solar collectors (PTSC) due to the fluid’s low cost, availability and thermal stability at high temperatures. However, the lower thermal transport properties of sCO2 compared to conventional working fluids necessitate heat transfer enhancement of receivers using sCO2. In this study, a combined and extensively validated approach using Monte-Carlo ray tracing for optical analysis and computational fluid dynamics (CFD) for thermal and thermodynamic analysis were used to investigate the overall thermal and thermodynamic performance of a high concentration ratio sCO2-based PTSC with a modified twisted tape (MTT) inserts in the receiver. MTT inserts with twist ratios in the range of 2.0 to 4.0 and width ratios in the range of 0.75 to 0.95 were investigated. Results show enhancement of the heat transfer performance by up to 73% and thermal efficiency by 6%, and reduction of the receiver heat losses and circumferential temperature gradients when the MTT inserts are used. In addition to presenting results using the first law of thermodynamics, this study also considered the second law of thermodynamics by evaluating the entropy generation rates due to heat transfer and fluid friction irreversibilities. Results showed that the use of MTT inserts caused a reduction in the total entropy generation rates, with the lowest irreversibility occurring at a Reynolds number of 1,650,000 for a twist ratio of 2.0 and width ratio of 0.95.

Shape matters: Convection and entropy generation in magneto-hydrodynamic nanofluid flow in constraint-based analogous annular thermal systems

This work analyzes the performance of annular thermal systems of distinct geometries to understand their shape impacts using a novel constraint-based methodology. This research fills the gap in the literature by analyzing the geometrical shape impact of magnetohydrodynamic CuO/water nanofluid flow on fluid flow, heat transfer, and thermodynamic irreversibility. Through computational simulations, the outcome results demonstrate that the circular annuli (Case 1 and Case 2) outperform the analogous square system in terms of fluid velocity, heat transfer, and irreversibility. The analysis is carried out for a range of Rayleigh numbers (Ra = 103 to 106), nanoparticle concentrations (ζ = 0 to 4%), magnetic fields (Ha = 0 to 100), and magnetic field inclinations (γ = 0° to 180°). Additionally, a significant improvement in heat transfer is observed by augmenting nanoparticles of the working fluid, while the magnetic field intensity plays a crucial role in controlling fluid flow and heat transfer. The study provides valuable insights into how various parameters affect the nanofluidic heat and flow characteristics of equivalent annular geometries and contributes to the development of more efficient and effective designs of thermal systems. The novelty of the study lies in its comprehensive approach in the use of a constraint-based methodology that matches the flow areas or cooling lengths while maintaining the same heating length to analyze the impact of geometric shapes on heat transport and accompanying irreversibility during MHD nanofluid flow. Furthermore, the energy-flux vectors are utilized to delve into the insights. The maximum value of heat transfer noted for Case 1 and Case 2 is approximately 7% and 11%, respectively, compared to the square annulus (Base Case). The inclusion of nanoparticles in the carrier fluid causes a significant enhancement in heat transfer by up to 11%, compared to pure fluid (ζ = 0).

A numerical simulation of the hydrothermal characteristics of a microchannel using cavities, ribs, and secondary channels

To tackle the overheating problem in microelectronic devices, microchannel heat sinks (MCHSs) are promising solutions. However, there is a need to improve their thermal performance while maintaining an acceptable pressure drop. This research investigates the effect of integrating multiple passive techniques for heat transfer enhancement, specifically examining the impact of combining triangular cavities, streamlined ribs, and oblique secondary channels (TC-SR-SC) on hydrothermal characteristics across a range of Reynolds numbers 100 to 600. By exploring the influence of each technique and their combining effects on fluid flow and heat transfer, the study aims to strike a balance between high thermal performance and low-pressure drop. Furthermore, the research also explores the thermal resistance, overall performance, entropy generation, and irreversibility of the novel design, comparing it numerically with commensurate geometries using the commercial software, FLUENT. The results reveal that the design of isosceles triangular cavities, streamlined ribs, and secondary channels combination (TC-SR-SC) achieves a maximum overall performance of 1.70 at Re = 400 due to the interruption and redevelopment of the thermal boundary layer. The presence of oblique secondary channels leads to the diversion of mainstream flow, resulting in better flow mixing between main adjacent channels. It also represents the lowest flow and heat transfer irreversibility, ultimately improving thermal performance based on the second law of thermodynamics perspective.

Thermal damping of the motion of a piston: Any irreversibility implies dissipation

We consider the motion of a frictionless piston that separates the surrounding atmosphere from an ideal gas enclosed within a cylinder, with no friction or viscous dissipation arising within the gas or surrounding atmosphere. Although no mechanically based dissipative mechanisms act, the motion of the piston is still damped if heat transfer between the gas and the piston occurs at a finite rate. Hence, as long as some kind of irreversibility develops within the system, such as irreversible heat transfer, the piston will not oscillate back-and-forth indefinitely and must eventually come to rest. We provide detailed thermodynamic and numerical analyses of this thermal damping, various aspects of which should prove useful to both students and instructors when discussing the first and second laws of thermodynamics.

Multivariate multi-objective collaborative optimization of pumped thermal-liquid air energy storage

Pumped thermal-liquid air energy storage (PTLAES) is a novel energy storage system with high efficiency and energy density that eliminates large volumes of cold storage. In this study, three different configurations of PTLAES systems with direct and indirect thermal energy storage were proposed. The “adaptive segmentation-based temperature transfer matrix” method was developed to capture the heat transfer characteristics of multi-stream plate-fin heat exchangers (MPFHEs), which is the key component of the system. To determine the upper limit of the techno-economic performance of the system, single/multi-objective optimization was performed with up to 35 decision variables. Additionally, the “Pareto efficiency” concept was defined to quantify the goodness of a point compared to the Pareto front. The results showed that the irreversible heat transfer and pressure drop in MPFHEs contribute 20.6 %–44.3 % of the total exergy destruction. Therefore, the design of MPFHEs significantly impacts the system performance. Multivariate multi-objective collaborative optimization can significantly improve the optimization results. PTLAES with closed loop indirect thermal energy storage was determined to have the best overall performance, achieving round-trip efficiency of 63.3–70.1 %, levelized cost of storage (LCOS) of 0.162–0.181 $/kWh, and energy density of 122–161 kWh/m3. The optimized PTLAES system can achieve lower LCOS than lithium-ion batteries when the discharge duration exceeds 3.5 h. Furthermore, it was shown that key criteria including round-trip efficiency, LCOS, and energy density can be traded off over a broad design space of different system configurations, parameter combinations, and material choices. This research demonstrated that PTLAES is a promising choice for long-duration energy storage and provided guidance for the design and techno-economic optimization of PTLAES systems.

Theoretical analysis of cavern-related exergy losses for compressed air energy storage systems

The paper presents a thermodynamic analysis of the exergetic losses occurring due to pressure and temperature variations within constant-volume compressed air caverns. Direct cavern losses are due to mixing and irreversible heat transfer, and may also include an exit loss as a result of unusable thermal exergy in the discharge air. Additional cavern-related losses may occur in other system components, including compressors (due to off-design operation), throttles and thermal stores. These indirect losses are also discussed and analysed for a simplified but representative adiabatic compressed air energy storage system. The overall aim is to determine trends in the various loss components with operating parameters (chiefly the minimum and maximum cavern pressures) and other thermal parameters. A comparison between isobaric and isochoric storage is also made and reveals the trends of efficiency vs. storage density for these two modes of storage.

Thermal management for multi-cores chips through microchannels completely or incompletely filled with ribs

Microchannels have attracted significant attention in the thermal management of integrated circuits due to their small geometrical size and high heat transfer efficiency. Motivated by further improving the thermal management capacity of microchannels in multi-cores chips with lower pressure drop penalty, in this paper, a comparative investigation on four different configurations was conducted through numerical simulation for the Reynolds number ranging from 200 to 800 to analyze the comprehensive effects of filling mode and rib arrangement on the flow and heat transfer characteristics, including microchannel completely filled with aligned ribs (MCFAR), microchannel incompletely filled with aligned ribs (MIFAR), microchannel completely filled with staggered ribs (MCFSR) and microchannel incompletely filled with staggered ribs (MIFSR). Moreover, the entropy generation rate is used to estimate the irreversibility of flow and heat transfer based on the second law of thermodynamics. Finally, the performance evaluation criteria (PEC) is adopted to estimate the overall performance. The results show the flow patterns in the MIFAR and the MIFSR are different from those in the MCFAR and the MCFSR due to the incomplete filling mode, which could induce more dramatic disturbance and further break the downstream boundary layer, thus the heat transfer could be enhanced with a lower pressure drop penalty. The analysis of entropy generation rate indicates that entropy generated by heat transfer contributes the majority of the total entropy generation thus enhancing the heat transfer is a more important and effective approach in reducing the irreversibility. Among the four configurations researched, the microchannel MIFAR shows the best overall performance (PEC = 1.76) when Re = 400.

Investigation of non-ideal effects in compressible boundary layers of dense vapors through direct numerical simulations

In this work, we present an investigation about the sources of dissipation in adiabatic boundary layers of non-ideal compressible fluid flows. Direct numerical simulations (DNS) of transitional, zero-pressure gradient boundary layer flows are performed for two fluids characterized by different complexity of the fluid molecules, namely, “air” and siloxane MM. Different sets of thermodynamic free-stream boundary conditions are selected to evaluate the influence of the fluid state on both the frictional loss and the dissipation mechanisms. The thermophysical properties of siloxane MM are calculated with a state-of-the-art equation of state. Results show that the dissipation due to both time-mean strain field, irreversible heat transfer, and turbulent dissipation differs significantly depending on both the molecular complexity of the fluid and its thermodynamic state. The dissipation coefficient calculated from the DNS results is then compared against the one obtained using a reduced-order model (ROM), which solves the two-dimensional boundary layer flow equations for an arbitrary fluid [M. Pini and C. De Servi, “Entropy generation in laminar boundary layers of non-ideal fluid flows,” in 2nd International Seminar on Non-Ideal Compressible Fluid Dynamics for Propulsion and Power (Springer, 2020), pp. 104–117]. Results from both the DNS and the ROM show that low values of the overall dissipation are observed in the case of fluids made of simple molecules, e.g., air, and if the fluid is at a thermodynamic state in the proximity of that of the vapor–liquid critical point.