International Journal of Thermal Sciences最新文献

The Oberbeck–Boussinesq equations and boundary conditions resulting from the conservation laws and thermodynamics principles provide the basis for mathematical modeling of evaporative convection in a bilayer liquid–gas–vapor system. The processes of fluid dynamics and heat and mass transfer in the volume phases and through the interface are successfully described with the help of a partially invariant solution of the constitutive equations. The solution is the efficient tool for studying regularities of physical phenomena as well as for describing heat-mass exchange processes with respect to the Ludwig–Soret mass transport and the diffusion thermoeffect appeared in the gas phase due to the presence of a volatile component. An exact solution of convection equations is derived under the assumption that evaporation/condensation is a process of the diffusive type and has an inhomogeneous character along the interface. Based on the comparison of the calculated and experimental values of the evaporation mass flow rate, the correct problem statement is specified that provides acceptable qualitative and quantitative agreement. The influence of the kinematic characteristics of the gas on the parameters of convective regimes arising in a horizontal mini-channel is investigated within the frame of the selected problem statement for the ethanol–air fluid system under the terrestrial gravity field. The topological structure of the bilayer flows, pattern of the temperature and vapor concentration fields, evaporation rate variations as well as the stability of the convective flows are analyzed with respect to different values of the gas flow rate. The destabilizing influence of the pumping gas on the threshold characteristics of the stability for the two-layer flow is ascertained. Three different wave modes of instability are predicted.

Polycarbonate panels (PC panels) are state-of-the-art transparent insulating materials widely used in the construction industry due to their cavity structure, which provides exceptional thermal insulation and optimal optical performance. However, the inherent anisotropy of the three-dimensional cavity structure complicates radiative transfer and requires consideration of both azimuth and zenith angles in optical performance evaluation. This aspect has received limited attention in existing research. This study aims to accurately characterize the optical performance of PC panels through numerical simulations. A three-dimensional radiative transfer model based on the discrete ordinate radiation model is developed to solve the radiation transfer equation. The model's independency regarding mesh division, angular discretization, and accuracy is validated. The effects of incidence angle, geometric parameters, and optical properties of PC panels on optical performance are analyzed. The findings reveal a strong correlation between transmittance and absorption with variations in incident zenith and azimuth angles. The transmittance exhibits a consistent monotonic variation expressible as a rational bifunction. Notably, absorption peaks occur within specific solid angle ranges, with increased structural complexity resulting in heightened absorption and greater uncertainty. For conventional PC materials, maximum transmittance ranges from 46.9 % to 73 %, while maximum absorption ranges from 2.3 % to 13.5 %. Increasing absorption coefficients, refractive index, and surface scattering coefficients nonlinearly decrease transmittance while increasing absorption. Additionally, deviations in transmittance and absorption with azimuth angle amplify with an increase in non-horizontal structures. Sensitivity analysis indicates a significant influence of zenith angle on transmittance, and absorption coefficient predominantly affects absorption.

Experiments were carried out to study hydrodynamics and heat transfer in a channel with discrete roughness elements on walls. Semicircle ribs with the height of 1.4 % of the hydraulic diameter of the channel were considered. Various rib geometries and positions were examined: solid ribs and ribs with slits, various streamwise and spanwise pitches, inline and staggered rib arrangements. Heat transfer enhancement and thermal-hydraulic performance were estimated. Integral and local values of the heat transfer coefficient were measured. Optical measurements yielded the fields of velocities and turbulent characteristics of the flow. Simultaneous analysis of hydrodynamic and thermal parameters of flow was carried out. Formation mechanisms of the kinematic structure of flow and heat transfer on roughened walls were considered. The distribution of heat transfer coefficient was shown to depend on the turbulent structure of flow, namely the vertical fluctuation of velocity in the near-wall region.

The study examines the coupling effect between air temperature and time-varying ventilation wind speeds within the tunnel, abstracting their influence on tunnel coldness. This investigation introduces a novel indicator—the equivalent mean air temperature within the tunnel—derived through fluid dynamics and heat transfer theories based on the principle of equivalent convective heat transfer. Case studies using the Xinjiaodong Tunnel and BSLL Tunnel illustrate the indicator's applications, including optimal anti-freezing axis identification, insulation layer thickness design, and active-controlled ventilation implementation. The optimal anti-freezing axis orientation angle for the Xinjiaodong Tunnel entrance section is 133.3°, deviating significantly by 160.8° from the actual axis, indicating a lower level of equivalent mean annual air temperature (5.5 °C) at the entrance section. This underscores the necessity to reinforce anti-freezing measures specifically at the entrance section of the Xinjiaodong Tunnel. Determining a 10 cm-thick insulation layer requirement at the Xinjiaodong Tunnel entrance section based on the equivalent mean air temperature. Through on-site investigation and published findings, it was observed that a 5 cm-thick insulation layer failed to prevent freezing, resulting in water leakage and ice formation on the lining, thus validating the calculation results. The BSLL Tunnel requires an insulation layer thickness exceeding 10 cm based on the equivalent mean air temperature, necessitating the implementation of active-controlled ventilation. Calculation results reveal that, with active-controlled ventilation wind speeds increasing from 1 m/s to 4 m/s at a temperature threshold of 4 °C, the equivalent mean air temperature during cumulative negative temperature periods within the BSLL Tunnel rises sharply from 1.0 °C to 2.7 °C. These findings demonstrate that the equivalent mean air temperature not only guides the identification of optimal anti-freezing axis, the design of insulation layer thickness considering time-varying ventilation wind speeds, and the implementation of active-controlled ventilation but also provides new methods and technologies for anti-freezing design in cold-region tunnels.

In the design of the nuclear rocket engine nozzles, the heat in the throat area of nozzles is so intense that the temperature may exceed the endurance of existing materials. Regenerative cooling technology is a widely employed method for effectively cooling the nozzle. The influences of a strategy involving the arrangement of ribs in the regenerative cooling channel are investigated. The flow and heat transfer characteristics in the regenerative cooling channels with three different types of ribs are compared and analyzed. In accordance with the computational results, two composite structures are proposed. Moreover, the flow and heat transfer characteristics in the bending channel with these two composite structures are calculated and analyzed. The results show that the straight and streamlined ribs can reduce the maximum temperature only when the height is lower than 1 mm. The side rib can reduce the temperature near the rib. For the composite rib, when the distance between the side rib and bottom rib is ranging from 3 cm to 5 cm, the appearance of the high temperature zone, which is located near the bottom rib can be prevented. Moreover, in the bending channel, the straight composite rib can considerably reduce the maximum temperature in the throat area by 96 K. However, the streamlined composite rib can only reduce the wall temperature near the rib.

Flow boiling instability stands as one of the key factors hindering the engineering application of large-area heat sinks with parallel multi-minichannels. In this study, a novel large-area heat sink with one inlet and two outlets was fabricated with aluminum alloy. It included 50 parallel minichannels, which were interconnected by the micro-slots arrays to balance the fluid pressure and redistribute the two-phase flow during flow boiling. The working fluid selected for the study was R134a, upon which flow boiling experiments were subsequently carried out. The effects of the thermal-hydraulic parameters, such as pressure drop across the test section, inlet and outlet pressures, and wall temperatures, on the two-phase flow state during flow boiling were analyzed. The flow boiling instability criterion for large-area parallel multi-minichannels heat sink was proposed, and the flow instability map was created to predict the two-phase flow state. It was found that the ratio of heat flux to mass flux (q_w/G) was an important parameter for stable and unstable two-phase flow. The flow boiling instability first locally appeared in the region near the outlet and then expanded to the entirety of the heat sink's flow passage with the escalation of the q_w/G ratio. In addition, the characteristics of heat transfer and the transitions in flow patterns during flow boiling instability were also discussed.

The present paper, for the first time, examines the influences of utilizing oil-ZnO nanofluid with different volume fractions φ = 0.5 %, 1 %, and 2 % in alternating flattened tubes (AFTs) with different alternating pitch angles of 30°, 45°, 60°, and 90° on the performance of a dual-tube heat exchanger (DTHE). This work is conducted experimentally and numerically for the Reynolds number (Re) range of 300 < Re < 1900 for oil-ZnO nanofluid and Re = 2000 for water. Based on the experimental results, the optimal case is selected for the numerical simulations of AFTs. The performance evaluation criterion (PEC) is defined for the simultaneous evaluation of pressure drop (Δp) and heat transfer coefficient (HTC). The results demonstrate that the overall heat transfer coefficient (U) and Δp are augmented with the inlet flow rate and the alternating angle between the pitches. Therefore, the maximum heat transfer (HT) and Δp correspond to the AFTs with the angle of 90° (AF4) at Re = 1900. The PEC amount of AF4 shows a 56 % enhancement compared to the circular tube. It is also observed that using copper oxide nanoparticles inside the oil improves the HT rate and Δp in the heat exchanger. Besides, an increment in φ increases U and Δp; however, the values of PEC show that the positive effects of the nanofluid are larger than their negative impacts in such a way that the PEC is improved by 64 % when the nanofluid with φ = 2 % is utilized in AFTs compared to the circular tube.

Nanoparticles (NPs) have attracted much attention recently because of their excellent photothermal properties. In particular, nanofluids (NFs) based on core-shell plasmon NPs have become the key to solar thermal utilization. This work proposed an ZrC–Au core-shell NP suitable for direct absorption solar collectors (DASCs). The optical properties of ZrC–Au core-shell NP are investigated based on the finite element method (FEM). The physical mechanism of its existence can be explained by the surface plasmon resonance and localized surface plasmon resonance of ZrC–Au core-shell NP. Meanwhile, the effect of core-shell size on the NP optical properties of ZrC–Au core-shell is investigated based on electromagnetic field distribution. In addition, the effects of length (H) and mass flow (ṁ) on the temperature rise and efficiency of the collector (η) are analyzed with DASC 2D simulation. Research shows that ZrC–Au core-shell NPs with t = 5 nm and r₃ = 15 nm can effectively broaden the solar spectral absorption band, increase the absorption peak value, and the photothermal conversion efficiency (f_v = 20 ppm, h = 15 mm) reaches 96 %, which is 15.89 % higher than Au NP. Meanwhile, the η of ZrC–Au NPs can be improved by ∼8.86 % compared with Au NPs under specific parameters (H = 2 cm, L = 20 cm, f_v = 20 ppm, ṁ = 1 g/(s m)). Combined with the preparation possibility and economy of ZrC–Au core-shell NPs, the broad application prospect of this NP in DASC and other photothermal fields was analyzed.

When liquid fuel leaks onto a busy road tunnel, it initially spreads in a two-dimensional manner, leading to a two-dimensional spill fire if ignited. As the tunnel width increases, the two-dimensional spreading and burning process becomes more enduring. This study aims to investigate the impact of tunnel width on flame dynamics, thermal feedback mechanisms, and heat loss mechanisms in ethanol spill fires. The results indicate that as the tunnel width increases, both the maximum combustion area (MCA), the increase rate of combustion area (IRCA) and stable combustion area (SCA) increase, but heat release rate per unit area (HRRPUA) decrease. As the channel width expands, the flame plume height and flame oscillation frequency rise. A new flame oscillation model considering flame shape ratio is introduced. Both channel width and flame length-to-height ratio influence flame oscillation behavior. Heat transfer analysis reveals that when the discharge rate increases and the width is between 0.05 m and 0.15 m, the fraction of radiant thermal feedback ${\chi }_{\text{rad}}$ significantly increases with discharge rate. However, when the width is between 0.2 m and 0.3 m, the ${\chi }_{\text{rad}}$ difference between different widths is minimal. Under the same discharge rate, as the channel width expands, both the convective thermal feedback ${\chi }_{\text{conv}}$ and heat loss ${\chi }_{\text{loss}}$ fractions tend to rise. A novel dimensionless burning rate model is developed, when the dimensionless heat release rate $Q_{hrr}^{*}$ increase, the burning rate ${m^{″}}^{*}$ exhibit an upward trend, albeit at a reduced growth rate. Once $Q_{hrr}^{*}$ reaches a sufficient magnitude, ${m^{″}}^{*}$ stabilizes at a constant value.

In the present experimental study, two distinct configurations of double layer microchannel heat sinks (DL MCHS) are proposed. It consists of rectangular parallel channels in the bottom layer and an array of square pin fins in the upper layer. Pin fin height $\left(H_f\right)$ in the first configuration is equals to the channel height ($H_c$) (i.e. $H_f/H_c=1$). However, in the second configuration, pin fin height is equivalent to 75 % of $H_c$ such that $H_f/H_c=0.75$. The proposed modified DL MCHS configurations are then compared with the conventional double layer microchannel heat sink (CDL MCHS). Hence, the idea of the current work is to develop a novel DL MCHS with improved heat transfer capabilities and reduced pressure penalties. Heat dissipation rate, pressure data and coolant flow behaviour are carefully measured and analyzed. Findings reveal that thermal performance of the modified heat sink with $H_f/H_c=1$ is not appreciable because it delivers almost similar to conventional configuration. Whereas, modified heat sink of $H_f/H_c=0.75$ has consistently exhibited better heat transfer rate and thermal performance factor. As compared to CDL MCHS, thermal performance factor was found ≈38 % higher in this case. Overall thermal and hydraulic performance of the DL MCHS is significantly influenced by the flow pattern of the coolant, which is a result of the novel channel design.