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This article proposes a predictive static model to evaluate the thermal-hydraulic performance of a shell and plate particle-SCO2 moving packed bed heat exchanger, with consideration of pressure drop, radiation and property variations. The results reveal that convection resistance on the SCO2 straight channel side occupies a large part of total thermal resistance in most design and off-design cases. A three-dimensional model inheriting conclusions from the static model is raised, which demonstrates that extremely short entrance region in the x-y plane hinders the overall heat transfer behavior of the SCO2 straight channel. Deservedly, suppressions of the development of thermal layers in the y direction will contribute to improving the thermal-hydraulic performance of the shell and plate particle-SCO2 moving packed bed heat exchanger. Four different fin configurations on SCO2 channel side are presented, analyzed and compared via the static model at the design-point case. Offset rectangular fins configuration behaves the best heat transfer enhancement in total heat transfer rate and overall heat transfer coefficient when compared with the original channel, followed by zigzag fins configuration, offset airfoil fins configuration and s-shaped fins configuration. Zigzag fins-modified channel expresses the worst hydraulic behavior, of which the pressure drop is nearly four times that of original straight channel. The pressure drop of channel with s-shaped fins nearly equals to that of the unmodified channel. Introduction of fins on SCO2 channel side reduces the ratio of convection resistance as a result of apparently increased SCO2 convection coefficients.

The thermal conductivity of a formation, which is a basic parameter in well logging, is difficult to measure when the formation is porous or fractured and filled with water. In order to describe the heat transfer in an aquifer, a notion of overall thermal conductivity was proposed, which includes the thermal conduction of the soil or rock and the advection of groundwater. This paper adopts the fractal theory to establish a geometric model of the aquifer, which helps with the analysis of the heat transfer mechanism in an irregular configuration, and derives a correlation of the overall thermal conductivity from the thermal resistance structure. The overall thermal conductivity of typical aquifers indicates that the advection in the aquifer cannot be ignored even if the Reynolds number (Re) of the groundwater is less than 1. However, when the velocity and temperature of groundwater are determined, the advection does not depend on the porosity of the aquifer and therefore can be regarded as a constant. Furthermore, this parameter provides an easy but effective way to analyze the heat exchange between the wellbore and the surrounding formation, especially in the ground source heat pump (GSHP) system.

This experimental study deals with the basic stability behaviors of CO2 natural circulation loop (NCL) in the critical region. Supercritical natural circulation loop system was carefully established and tested, in which loop strong natural circulation can be achieved due to densities differences of the heater and cooler. Compared with normal fluid based NCLs, trans-critical and supercritical fluid based NCLs can sustain much higher flow rate and heat transfer capacity. In the current study, effects of operation parameters (charging mass, heating power and coolant temperature) on stability and flow transitions are investigated. Three types of trans-critical flow condition in high pressure NCL are identified and analyzed in this study. It is found that the two-phase flow or single-phase gas flow at sub-critical region may lead to instability. However, for supercritical condition in the current tests, all cases show single-directional steady behavior. The stability evolutions along with the variations of operating conditions are also analyzed for each case. The results for basic near-critical fluid NCL behaviors and rules of parameter dependencies can be useful for understanding of related systems.

Magnetic fluids are thermo-sensitive and whose flow and energy transport processes can be controlled by temperature and external magnetic field. In the natural convection of porous cavity, magnetic force is not only the driving force, the effective gravity is also a force related to the natural convection, and the effective gravity is closely depending on the porosity and permeability of porous medium. As is known, the porous medium is in solid state with high heat capacity but low heat transfer coefficient, while the magnetic fluid is a kind of fluid with high heat transfer coefficient and easy to be controlled. Combining the complementary characteristic of magnetic fluids and porous medium, we present a study for temperature-sensitive magnetic fluids in a porous square cavity. In the study, a Lattice Boltzmann method is developed to simulate the laminar convection of temperature-sensitive magnetic fluids in a porous square cavity. We present numerical results for the streamlines, isotherms, magnetization for different values of porosity and Darcy number. In addition, Nusselt numbers on heated and cooled wall and the average Nusselt numbers are also investigated.

Heat transfer characteristics of supercritical CO2 (S-CO2) fluid in collectors of a solar Rankine cycle system is experimentally investigated at different flow conditions. In the experiments, temperature, pressure and mass flow rate are measured, and local and average heat transfer coefficients are investigated. Study shows that the thermophysical properties of CO2 near the critical point have significant effects on the S-CO2 local heat transfer behaviors. The heat transfer of S-CO2 is also dependent on the mass flow rate, input heat flux and pressure at inlet of test section.

A solar collector using supercritical CO2 as working fluid is proposed in this paper. In order to investigate and estimate the CO2-based solar collector, an experimental set-up was constructed. Of particular interest of this paper are the basic collector characteristics, such as CO2 temperature and pressure in the collector, CO2 flow rate, and collector performances. The collector has been tested under various weather conditions. The results show that the CO2 temperature, CO2 pressure and mass flow rate increase with the solar radiation, which is different from those of traditional solar collector using liquid as working fluid. The solar radiation has influence on the CO2 states, being liquid, liquid-gas or supercritical state in the test, furthermore, affects the CO2 mass flow rate. The annually-averaged collector efficiency is found to be above 60.0% in the case of supercritical CO2 as working fluid, which is much higher than that of water-based solar collector. This study shows the potential of the supercritical CO2-based solar collector in the field of solar thermal utilization.