This special issue is based on a selection of papers presented at the 9th International Symposium on Advances in Computational Heat Transfer (CHT-24), sponsored by the International Centre for Heat and Mass Transfer (ICHMT) and cosponsored by Istanbul Technical University (ITU), Rutgers University, and the American Society of Thermal and Fluids Engineers (ASTFE). It was held at ITU, Istanbul, Turkey, during May 26-30, 2024. This symposium is a continuation of a series of symposia held at various places around the world, including Cesme and Izmir, Turkey; Marrakech, Morocco; Bath, England; Piscataway, New Jersey, USA; Napoli, Italy; and Rio de Janeiro, Brazil. It focuses on computational heat transfer and covers a wide range of topics of both fundamental and applied interest.
Analytical solutions describing heat transfer in moving bed heat exchangers (MBHEs) have been previously identified through Laplace transforms. This work employs an integral transform (IT) technique to obtain the analytical solution for a counter-current parallel-plate MBHE configuration, for a heat capacity ratio less than one. The development delineates the decoupling steps for the problem, along with the identification of the auxiliary problem's orthogonality property-which is necessary to obtain a solution via ITs. As expected, the solid and fluid temperature functions identified are identical to those in the literature. A graphical analysis is also presented, illustrating the solution's behavior. For example, at a number of transfer units (NTU) of 1, the average solids outlet temperature only drops to approximately 0.5, whereas at an NTU of 10, it approaches the fluid's inlet temperature, demonstrating the exchanger's capacity to reach thermal equilibrium. Overall, the integral transform technique provides a robust approach to solving counter-flow MBHE problems and opens the door for exploring more complex configurations.
Lack of knowledge about bubbly flow development in vertical channels is identified. An experimental setup is built for two-dimensional, two-component particle image velocimetry (PIV) and bubble shadowgraphy measurements of gas fraction, liquid-phase velocity, and gas-phase velocity profiles. Axial development of the gas phase injected coaxially into the upward water flow from a point source is explored. The developing region of two-phase flow is defined based on the axial position where the bubble core spreads over the cross section. This definition is found to be consistent with a prior work and is shown to be consistent with the behavior of the main kinematic parameters. The axial distributions of the bubble core width, gas fraction, peak liquid-phase velocity, and peak gas-phase velocity in the developing region demonstrate initially oscillatory behavior followed by an asymptotic one. The axial distribution of the liquid-phase velocity has a prominent, elongated peak closer to the gas injection level. Its position is independent of the flow conditions. The rest of the flow parameters also have prominent extremums, positions of which are independent of the flow conditions. The span of the developing region does not show significant dependence on the flow conditions. It is suggested that the pressure head, which was not changing during the experiments, could have the main influence on the mentioned extremums. Two methods of slip velocity calculations are suggested; their advantages and disadvantages are explored. Only one method is shown to capture all the aspects of the kinematics of the developing region. An attempt is made to consider gas injection from the perspective of the jet theory. It is shown that the existent methods fail to predict the main kinematic parameters and that the influence of both gas phase and liquid phase inlet parameters must be taken into account, just as is done in the developed region.
In the present study, the effect of different linear heating and salting profiles on the validity of the local thermal non-equilibrium (LTNE) approach within a cavity composed of two distinct layers, a porous layer and a clear binary fluid layer, is investigated numerically. Entropy generation is employed as an analysis tool, and simulations are performed using the finite volume method and an in-house FORTRAN code. The influence of several key parameters on internal irreversibility and the LTNE state is examined. These parameters include the heating and salting profiles, buoyancy ratio (N), dimensionless porous layer thickness, and the porosity of the porous layer (epsilon). It was observed that, depending on the temperature and concentration profile, a composite cavity with substantial porous layer thickness evolves into a significant source of entropy generation, exhibiting a particularly high level of LTNE between the two phases of the porous layer.
Effective cooling of electronic systems in electric vehicles is critical for maintaining thermal performance and reliability while minimizing system weight and complexity, particularly in compact configurations like two-pass channels, where flow distribution and thermal efficiency are challenging. In this study, the passive modification of a two-pass cooling channel was investigated using various types and sizes of interconnecting slot configurations. The interconnecting slots were installed in the divider wall to direct coolant to the downstream channel leg, improving the cooling efficiency for onboard circuits on the channel top. The simulations were conducted at a Reynolds number of 1.2 & times; 104, based on the channel hydraulic diameter of Dh = 26.3 mm. The simulations employed Reynolds-Averaged Navier-Stokes (RANS) modeling with the shear-stress transport (SST) kappa-omega turbulence model. Two slot designs, including straight and converging interconnecting slots with three slot sizes of 0.08Dh, 0.16Dh, and 0.24Dh, were analyzed. The introduction of interconnecting slots yielded significant performance improvements. Hydrodynamically, the slots substantially reduced the channel pressure drop compared to the no-slot baseline by up to 65% for the largest slot and 36% for the smallest. Thermally, the maximum module temperature slightly decreased with slot integration, while temperature uniformity improved, as shown by a 13% reduction in the inter-module temperature difference relative to the baseline. Notably, the straight and converging slot configurations achieved comparable levels of heat transfer enhancement and pumping loss reduction. However, the converging slot design requires more material removal from the channel wall, thereby reducing the overall weight of the cooling system while maintaining similar performance.
Ventilation plays a crucial role in optimizing microclimates within vertical farms, where plant leaves continuously exchange heat and water vapor with their surroundings. This study develops a computational fluid dynamics (CFD) model that couples heat and mass transfer at the leaf level. A numerical methodology is implemented to calculate the local leaf temperatures based on energy balance equations, accounting for transpiration-driven cooling. To verify the CFD methodology, discrete Green's functions (DGFs)-a proven analytical method for convective heat transfer-are extended to also model mass transfer, allowing for a coupled heat and moisture transport analysis. The verification process first compared CFD and DGF results in a 2D flat plate case, where relative errors remained below 2%. The approach was then extended to a 3D leaf geometry, confirming strong agreement, with errors staying under 5%. To assess model robustness, 96 simulations were performed, systematically varying airflow velocity, radiation intensity, and stomatal resistance. The results illustrate key interactions between environmental conditions and leaf transpiration, such as counterintuitive reductions in transpiration under increased velocity when radiation is present. This validated CFD framework provides a reliable tool for modeling plant microclimates and optimizing climate control in greenhouses and vertical farms. Future research will extend this approach to simulate full plant architectures and improve environmental regulation strategies in controlled agriculture.
Transperineal focal laser ablation is an emerging novel technique to treat prostate conditions, such as prostate cancer or benign prostatic hyperplasia (BPH). The latter, particularly, is highly prevalent among aging males and significantly compromises urinary and sexual functions. Minimally invasive treatments are gaining increasing interest in treating this disease. Lack of a standardized protocol to set the treatment duration and the amount of energy delivered to achieve the desired ablation volume makes it necessary to develop accurate mathematical models, allowing preliminary computations to set operating parameters correctly, enabling the development of tailored treatments based on patient-specific conditions. For these reasons, this study implements a mathematical model to simulate focal laser ablation for BPH via an exhaustive parametric analysis. The model is solved using a finite element commercial software, COMSOL Multiphysics. Given the approximately spherical shape of the prostate, the computational domain is reduced to a two-dimensional axisymmetric geometry to minimize computational time. The biological domain is modeled as a porous medium with two separate phases: solid (i.e., tissue) and fluid (i.e., blood), under the assumption of local thermal nonequilibrium. This approach solves the heat transfer equations separately for the tissue and blood phases. The variation of thermophysical properties with temperature and the resulting thermal damage are also accounted for. The heat source is modeled as a collimated laser beam, with incident intensity described by Beer-Lambert's law. Using the developed model, the effects of treatment duration [100-(15 x 102) s)] and laser power (one to ten watts) are extensively investigated, considering varying prostate dimensions (20-50 mm). This provides clinicians with valuable insights into treatment dosage. Additionally, polynomial functions (valid only withing the investigated parameter ranges, to avoid inaccurate predictions) have been derived for each prostate radius, offering an immediate tool to estimate thermal damage and its percentage over the entire prostate as a function of time and delivered power.
This paper presents a novel discovery of a symmetry-breaking effect in porous media with porosity between 0.8-0.9, which we are referring to as the intermediate porosity flow regime. Using large eddy simulation, we studied how heat transfer and turbulent convection occurs within these materials at a microscopic level. We observed symmetry-breaking in porous structures made of regularly spaced circular cylinders, a common design in heat exchangers, immediately following the laminar to turbulent flow transition between Reynolds numbers of 37 and 100. Asymmetric patterns persisted up to Reynolds numbers of 1,000. The initial breakdown of symmetry occurs through a Hopf bifurcation, creating an oscillating flow pattern as shear layers interact around the solid obstacles. When the flow becomes turbulent, random variations in the timing of vortex oscillations (caused by the secondary instability) create asymmetric distributions of fluid velocity and temperature throughout the porous space. This leads to the formation of alternating channels with high and low velocity fluid flow. At the macroscale level, this loss of symmetry creates residual transverse drag force components and asymmetric heat flux distribution on the solid obstacle surfaces. Interestingly, the oscillating flow pattern promotes attached flow on the circular cylinder surfaces, which enhances heat transfer from the cylinders to the fluid. We observe that this secondary flow instability is the primary mechanism of enhanced turbulent heat flux from porous media with circular cylinders compared to those with square cylinders.
The popularity of additive manufacturing has increased interest in using triply periodic minimal surfaces (TPMS) in engineering applications due to their parametric analytical representation and potential for superior mechanical, heat, and mass transfer properties. This study numerically and experimentally investigates the flow and heat transfer behavior of gyroid TPMS structures with varying geometric frequencies. The primary objective is to explore how frequency variations in the gyroid structure, while maintaining equal porosity, affect pressure drop and heat transfer characteristics. Two 3D-printed gyroid structures with equal porosity but different lateral frequencies were tested for pressure drop across a range of flow rates. Simultaneously, steady incompressible Reynolds-averaged Navier-Stokes (RANS) simulations using the k-epsilon turbulence model were performed to study internal fluid flow and heat transfer behavior. Simulation results demonstrated that high-frequency TPMS structures lead to a lower pressure drop across all flow rates while exhibiting similar heat transfer rates compared to their low-frequency counterparts. These findings are supported by an analysis of energy dissipation near and far from walls, revealing the role of curvature and geometric distribution in fluid resistance and thermal performance. Although numerical and experimental results show some deviation, consistent trends validate the approach. This research highlights the potential of high-frequency gyroid structures in applications demanding low pressure drop without compromising heat transfer efficiency, with implications for compact heat exchangers and thermal management systems.
In this paper, we investigate the behavior of different viscosity models under conditions of conjugate natural convection in a square cavity divided by a heat-conducting partition with a changing inclination angle and differential heating of the side walls. Numerical modeling is performed using the finite volume method for a fluid with a viscosity similar to the properties of blood. The main parameters of the study were the cavity inclination angle (0 degrees, 30 degrees, 45 degrees, 60 degrees, and 90 degrees) and the choice of the viscosity model of the non-Newtonian fluid. The results showed that inclination angles of 30 degrees and 60 degrees contribute to the enhancement of convection flows. Among the studied non-Newtonian viscosity models and the power law model demonstrated the highest flow velocity, whereas for the Herschel-Bulkley model, heat transfer due to conduction prevailed, with minimal mixing process.
