Abstract Direct contact condensation experiments of steam in subcooled water are carried out for subcooling levels of ΔTsub= 20 °C, 30 °C, and 40 °C with a steam injection mass flux of 20 kg/m2 s. Rainbow schlieren deflectometry is employed to visualize the thermal gradients around the condensing steam bubble in a nonintrusive manner. For the chosen flow and subcooling conditions, the bubbling regimes observed are the steam bubble growth stage, the bubble receding stage, and the bubble collapse stage. Two-dimensional images captured during the process of bubble condensation using rainbow schlieren images are presented. The redistribution of color captured through the recorded images directly reflects the thermal gradients present in the test section. Qualitative interpretation of the recorded images reveals that the thermal gradient layer thickness around the condensing steam bubble increases during the growth and receding stages, before the complete breakup of thermal gradient layer at the bubble collapse stage. The local profiles of hue distribution in the direction normal to the thermal gradient layer indicate high temperature gradients in this narrow region. The hue values and the average thickness of the thermal gradient layer were found to be maximum for 40 °C subcooling level compared to the other cases. The rate of growth and thereby the collapse of the thermal gradient layer is slower for low subcooling levels and increases with higher subcooling values. To the best of the knowledge of the authors, this work is one of the first attempts to simultaneously capture the dynamical parameters of the condensing steam bubble as well as the associated thermal gradients field using a single imaging technique, thus making the experimental approach relatively simple and cost effective.

Abstract Nondiffusive thermal transport in solids and their micro/nanostructures is a key subject in the research of micro/nanoscale heat conduction. A number of laser and optical techniques to measure or capture the nondiffusive behaviors of heat carriers have been developed, such as transient thermoreflectance, time-domain thermoreflectance (TDTR), transient thermal grating (TTG), and so on. Here, we propose a novel method to study micro/nanoscale heat transport, namely, speckled laser pump–thermoreflectance microscopy probe. In this technique, micrometer to few hundred nanometer size random heat spots are generated by a speckled laser pump pulse, and the time–space evolution of heat spots are recorded by thermoreflectance microscopy images of the probe pulses arriving at different delay times. Fourier transform is applied to analyze the thermoreflectance images and extract the thermal decaying time for different spatial frequencies and along different in-plane directions. Thermal conductivity at different spatial frequencies, which includes the nondiffusive transport information, is obtained in this way. By numerically performing simulation of anisotropic Brownian motion and solving phonon Boltzmann transport equations under the initial condition of random heat spots, we retrieve the preset anisotropic thermal conductivity and the nondiffusive behavior of reduced thermal conductivity with increasing spatial frequencies, proving the validity of this technique. The innovative method can also be applied to study electron and spin transports, and holds the potential to facilitate the experimental research and understanding of nanoscale energy transport.

Abstract This review provides a comprehensive overview of the recent progresses in flexible heat spreaders based on functionalized boron nitride nanosheets (f-BNNSs) and provides insights into the potential of this material for efficient thermal management solutions. The unique thermal properties of boron nitride nanosheets (BNNSs), such as high thermal conductivity and excellent mechanical flexibility, make them promising materials for heat dissipation. Various methods of functionalizing BNNSs to enhance their thermal and mechanical properties are discussed, including covalent functionalization and noncovalent functionalization. The numerous advanced methods to introduce f-BNNSs into polymers and the potential applications of the composites based on f-BNNSs in areas such as thermal management in electronic devices are also highlighted.

Abstract Heat transport through 3D printed fractal media is investigated by comparing a fractal diffusion model to infrared measurements using Bayesian uncertainty quantification. The Delayed Rejection Adaptive Metropolis (DRAM) algorithm, based on the Markov Chain Monte Carlo (MCMC) sampling technique, is used to infer parameter uncertainty, quantify parameter correlation, and compute error propagation of the temperature distributions. The results demonstrate that fractal operators improve modeling thermal transport through complex fractal structures and help understand fractal structure-property relationships. For example, correlations among fractal spatial and temporal scaling parameters, diffusion coefficients, and fractal dimensions are quantified. We find a scaling relationship between the diffusion coefficient D and the temporal fractal time derivative order α that scales nominally as D α eα based on constraints from the second law of thermodynamics. The results have implications for building a stronger understanding of heat transport in complex materials beyond random media and models based on Gaussian probability homogenization.

Abstract The current study is concerned with the numerical simulation of the phase change process in a two-dimensional Dual-Phase-Lag (DPL) bioheat model applied to nanocryosurgery. A Gaussian radial basis function meshfree approach coupled with a Crank-Nicolson type of time discretization is employed on an irregular soft tissue domain. The simulation considers the introduction of three types of nanoparticles?Gold, Alumina Oxide, and Iron Oxide into the cryosurgical process. Temperature profiles were computed for situations both with and without the incorporation of nanoparticles, and the freezing interface was analyzed under different conditions. The results demonstrate the significant influence of nanoparticles on enhancing the freezing process, leading to a more controlled and effective cryoablation. The inclusion of nanoparticles not only accelerates the freezing front but also provides a more uniform temperature distribution within the target tissue. This study highlights the advantages of using a meshfree RBF approach in handling complex geometries, alongside the potential of nanoparticle enhanced cryosurgery to improve clinical outcomes. These findings contribute valuable insights into the optimization of cryosurgical techniques and the development of more effective cancer treatments.

Abstract Linear temporal and spatial stability analyses of the magnetohydrodynamic boundary layer flow over a wedge embedded in a porous medium have been carried out to analyze the effects of pressure gradient, Hartmann and Darcy numbers. Firstly, we determine the base state velocity profiles by imposing suitable similarity transformations on governing boundary layer equations and then find linear perturbed equations involving Reynolds number and disturbance wavenumber using Fourier modes. The Chebyshev spectral collocation method which provides insight into the complete structure of the eigenspectrum is used. The effect of Hartmann and Darcy numbers on the boundary layer is to stabilize the flow for all adverse pressure gradient parameters while only unstable modes are noticed in the absence of these effects. The noticed unstable modes are always part of the wall mode for which the phase speeds are found to approach zero. The eigenspectrum for all involved parameters has a balloon-like structure with the appearance of wall mode instability. The critical Reynolds number is found to be increasing for increasing pressure gradient, Hartmann and Darcy numbers. For an adverse pressure gradient, the energy budget shows that the energy production due to Reynolds stress dominates the viscous dissipation which results in destabilization of the flow, while the kinetic energy due to magnetic field and porous medium plays a role in stabilizing the flow. The physical dynamics behind these interesting modes are discussed.

Abstract A local thermal non-equilibrium analysis was made to assess potential use of bi-disperse porous walls for a transpiration cooling system. A three-energy equation model successfully used for the transient thermal analysis of bi-disperse packed bed thermocline storage systems was introduced to investigate various heat transfer aspects of transpiration cooling through a bi-disperse porous wall made of combination of large and small particles. Three independent energy balance equations, namely, the energy equation of the coolant gas phase, that of the solid phase of large particles and that of small particles are coupled with one another to obtain a set of exact expressions for all three individual temperature distributions across the porous wall for given thermal boundary conditions of the third kind. It has been revealed that the solid wall temperature of the bi-disperse porous wall stays lower than that of the mono-disperse porous wall in the high Peclet number range, resulting in a higher overall cooling efficiency for a given blowing flow rate. Furthermore, the analysis provides a suitable range of the Peclet number, under which the transpiration cooling should be operated to suppress excessive heat loss to the coolant reservoir at the same time to ensure a high overall cooling efficiency.

Abstract The highly corrosive environment inside a Supercritical Water-Cooled Reactor places stringent demands on the fuel rod cladding material, particularly requiring it to have strong corrosion resistance. However, within a refueling cycle, an oxide layer is growing on the surface of the fuel rods. In any case, the heat transfer to water under supercritical pressure conditions is an overly complex phenomenon, since the thermophysical properties of the fluid show drastic variations with respect to the temperature around the pseudo-critical temperature. An increase in the surface roughness height has an impact on heat transfer. To provide insight into the effect of surface roughness on heat transfer an experimental database, using the surrogate fluid R134a, covering a range of flow conditions is established. The database consists of reference data, obtained in a conventional hydraulic tube and of data obtained in a tube with an artificially roughened inner surface. In the present work, the impact of the surface roughness on heat transfer is evaluated, comparing the results obtained in the smooth tube, to the results obtained in the tube with rough inner surface. Heat transfer is enhanced when the Reynolds number is large enough and heat transfer deterioration can be suppressed or shifted to larger bulk enthalpy, due to the roughness. Furthermore, existing empirical correlations are assessed against the newly generated experimental database. It is concluded that none of the available correlations satisfactory predicts the experimental data over the entire range of Reynolds numbers, surface roughness and wall-to-bulk temperature ratios.

Abstract This article studies the natural convection in an annular porous microchannel in case of one wall being heated and another being cooled. For the first time, such a problem was solved using discrete symmetries of the Navier-Stokes equations. Using discrete symmetries, self-similar forms of differential equations were obtained. Solutions of self-similar equations made it possible to obtain velocity and temperature profiles incorporating slip and temperature jump at the channel walls as boundary conditions. The effects of Grashof, Knudsen, Darcy, and Prandtl numbers on the velocity and temperature profiles in the microchannel and Nusselt numbers are demonstrated. At high Grashof numbers, an ascending flow forms near the hot cylinder, while a descending flow develops near the cold cylinder. As the Knudsen number increases, rise of velocity and temperature jump at the walls as well as heat transfer coefficients decrease is observed. An increase in the Darcy number results in higher velocities for both flows. The temperature jump at the heated cylinder increases, remaining unchanged at the cooled cylinder, and the heat transfer coefficient at the heated cylinder drops.

Abstract Hybrid fuel cells are becoming increasingly popular in 21st century energy systems engineering. These systems combine multiple features including various geometries, electromagnetic fluids, bacteria (micro-organisms), thermo-solutal convection and porous media. Motivated by these developments in the present work we simulate the two-dimensional magnetohydrodynamic (MHD) natural triple convection flow in a semi-trapezoidal enclosure saturated with electrically conducting water containing oxytactic microorganisms and oxygen species. Darcy?s model is deployed for porous media drag effects. The primitive governing partial differential conservation equations for mass, momentum, energy, oxygen species and motile micro-organism species density are transformed using a vorticity-stream function formulation and non-dimensional variables into a nonlinear boundary value problem. A numerical solution is obtained using a finite difference method with incremental time steps. The mathematical model features a number of controlling parameters i.e. Prandtl number, Rayleigh number, Bioconvective Rayleigh number, Darcy parameter, Hartmann (magnetic body force) number, Lewis number, Péclet number, Oxygen diffusion ratio, fraction of consumption oxygen to diffusion of oxygen parameter. Transport characteristics (streamlines, isotherms, oxygen iso-concentration and motile micro-organism concentration) are computed for several of these parameters. Microorganisms? impact on the rate of heat transfer at the boundaries is found to be beneficial or destructive, depending on combination of other parameters in the simulations. Additionally, Nusselt number and oxygen species Sherwood number are computed at the hot vertical wall. The simulations are relevant to hybrid electromagnetic microbial fuel cells.