Xin-Rong Zhang

Preliminary Study on a Solar Water Heater Using Supercritical Carbon Dioxide as Working Fluid

In this paper, a solar water heater using supercritical carbon dioxide as working fluid is proposed and experimentally studied. For supercritical carbon dioxide, a small change in temperature or pressure can result in large change in density, especially in the state close to the critical point. Thus, natural convective flow of the supercritical carbon dioxide can be easily induced by solar heating or water cooling. Such convective flow absorbs and transports heat to water in solar collector tubes. Motivated by the above idea, an experimental setup was designed, and a solar water heater was tested. The obtained results show that natural convective flow is well induced, and a flow of 1900 Reynolds number can be achieved even in winter, when the lowest level of solar radiation condition occurs. Furthermore, the measured collector and heat recovery efficiencies are 66.0% and 65.0%, respectively. More details of its mechanism are to be studied, and a complete performance analysis is needed.

Simulation of Heat Transfer and System Behavior in a Supercritical CO2 Based Thermosyphon: Effect of Pipe Diameter

This paper deals with the natural convective circulation thermosyphon with supercritical CO 2 flow. New heat transport model aiming at supercritical thermosyphon heat transfer and stability is proposed and numerically studied. Two-dimensional rectangular natural circulation loop model is set up and the effect of pipe diameter is systematically analyzed. Finite volume method is used to solve the conservative equations with supercritical turbulence model incorporated. It is found that supercritical CO 2 thermosyphon can achieve high Reynolds flow as 104―105 even temperature differences between source and sink is small. Stabilized flow is found for larger pipe diameter group due to the developed flow field and enhanced heat transfer. Heat transport at cooler side can be enhanced at higher operating temperature and be critical for the stabilization of the supercritical thermosyphon. Correlations of flow and heat transfer are reexamined and good agreements with classical reports are also obtained in the present study.

Effects of Heater Orientations on the Natural Circulation and Heat Transfer in a Supercritical CO2 Rectangular Loop

Supercritical fluid based heat transfer loop (NCL) has become a hot topic in energy conversion systems. In such systems, supercritical natural convection stability and heat transfer conditions are crucial for design and safe operation. In the present study, numerical simulations were performed to investigate the influences of heater orientations on the performance of supercritical CO2 based circulation loops. The numerical model is based on Navier-Stokes equations with supercritical turbulence effects considered. It is found that the heat source location has significant influence on the flow pattern and system heat transfer. Vertical heating cases are found stable in a wide range of heat flux conditions due to the changes of buoyancy force torques across the NCLs, while horizontal heating cases show transition heat flux and oscillations are still seen. However, the influence of heat source location is less significant on the heat transfer characteristics. The effect of cooler heat transfer is found of special importance for the heat transfer and system stability behaviors. The NCL flow and heat transfer correlations are also compared in this study, and it is recommended that more numerical and experimental studies be made in the future.

Laminar Natural Convection Heat Transfer From a Vertical Baffled Plate Subjected to a Periodic Oscillation

The problem of laminar natural convection on a vertical baffled plate subjected to a periodic oscillation is investigated numerically. Of particular interest of this paper is the heat transfer characteristic with the oscillatory velocity being close to the flow velocity in the velocity boundary layer under nonoscillation condition

Experimental Performance Analysis of Supercritical CO2 Thermodynamic Cycle Powered by Solar Energy

The interests in using carbon dioxide as working fluid increase since the Montreal and Kyoto Protocols were made. In this paper, a complete effort was made to study the performance of CO2 Rankine cycle powered by solar energy experimentally. The system utilizes evacuated solar collectors to convert CO2 into high‐temperature supercritical state, used to produce electrical energy and thermal energy, which could be used for air conditioning and hot water supply and so on. The system performances were tested not only in summer, but also in winter; not only in sunny day, but also in cloudy day. The interest of the paper is the solar collector efficiency, because the absorbed heat quantity in the collector can be utilized for power generation and heat supply and other useful outputs. The results show that annually‐averaged solar collector efficiency was measured at about 60.4%. The study shows the potential of the application of the solar powered CO2 cycle as a distributed power/heat generation system.

Damping characteristics and flow behaviors of an ER fluid with a piston sine vibration in a viscous damper

The damping characteristics and flow behaviors of ER fluids inside a piston–cylinder viscous damper subjected to external electric fields are studied based on experiment, theoretical analysis and numerical simulation. The viscous damper is a closed system with an inner piston and an outer cylinder, which is designed and constructed in our laboratory. In the experiment, the test ER fluid is enclosed in the gap of a piston–cylinder system. To examine the damping characteristics of the test ER fluid, a piston sine vibration experiment is performed with accompanying theoretical analyses. In addition, in order to investigate the ER flow behaviors inside the damper, a numerical simulation is carried out. The present study discloses the damping characteristics and the fluid mechanism of the ER fluid in the piston–cylinder damper with an applied external electric field.

Investigation of Impulse Response of an ER Fluid Viscous Damper

The electrorheological (ER) fluid is a fluid that shows non-Newtonian fluid characteristics when the electrical field is applied. In this study, an experiment of an ER fluid viscous damper, in which an ER fluid is enclosed in the gap of a piston—cylinder system, is carried out to investigate the performances of the ER fluids and the effects of the electric field on the impulse response of the damper when an impulsive force is applied to it. To validate the experimental observations, a theoretical cylinder displacement formula of the damper is derived based on the Bingham approximation and the Stokes flow assumption of the test ER fluids in the damper. The present study shows that, when strength of the applied electric field and concentration of the ER fluids increase, the oscillation amplitudes of the cylinder displacement reduce. In other words, the ER damping effect is improved when the strength of the electric field increases.

Development of Supercritical CO 2 Solar Rankine Cycle System

A supercritical CO2 solar Rankine cycle system, an innovation of a new concept for global warming solution by using CO2 as a natural working fluid is introduced and the development of the novel system is presented. The system consists of solar collectors, power generation turbine, heat exchangers, and mechanical feed pump (or a novel concept the so-called thermally driven pump). This system is particularly characterized by CO2 transcritical Rankine cycle with newly developed system elements, which include evacuated tube solar collector, turbine, gas–liquid heat exchanger, feed pump and other flow regulating elements. In this article much attention is given to the thermally driven pump, which shows promising performance data, when replacing for a mechanical feed pump in the system. Preliminary results gained from a prototype system installed for an actual operation under extracting solar thermal energy, producing electric and heat energy, indicate that the system has more advantages against a fossil fueled cogeneration system. The developed system represents highly potential solution and idea to solve the global warming crisis, and also can give a clue to a path of future energy creation technique for green energy resources.

Near-Critical Natural Circulation Flows Inside an Experimental Loop: Stability Map and Heat Transfer

The near-critical CO2-based natural circulation loop (NCL, or thermosyphon) has been proposed in many energy conversion systems, such as the solar heater, waste heat recovery, next-generation nuclear cooling, and so on. There is an increasing need to obtain detailed information about such systems, as it is less verified from a basic system operation viewpoint. This paper presents an experimental investigation of a near-critical CO2 thermosyphon. The closed thermosyphon is specially designed for high-pressure (in the critical region, from 6.0 MPa to 15.0 MPa), natural circulation flows. The basic transient flow behaviors and parameter behaviors are found to be dependent on initial pressure. The system stability evolution from subcritical oscillating flow to supercritical stable operations is presented. From the experimental data analysis, the stability map for the current supercritical natural circulation loop system is given. It is found that the stability pressure lines will divide the operation into stable, transition, and unstable regions. It is found that the effectiveness of the cooler will greatly affect the system stability, while the heat transfer efficiency is mainly controlled by the heater conditions. Parameter evolutions of the fluid temperature, mass flow rate, and loop pressure are presented in this paper. The heat transfer dependency on operation pressure and evolution mechanisms are also discussed in detail in this paper.

Bifurcated Forced Convective Heat Transfer of Supercritical CO2 Flow in Plane Symmetric Sudden Expansion Duct

This study presents a computational investigation of forced convection of supercritical CO 2 flow in plane symmetric sudden expansion duct at an expansion ratio of 2 in flow asymmetric regime. Computations were conducted at various Reynolds numbers in flow asymmetric regime and low wall heat fluxes below 500 W/m 2 to examine the Reynolds number and thermal effects on the flow and heat transfer of the bifurcated flow General flow features and temperature distributions are presented. The transitional Reynolds numbers above, which a third recirculation region will appear at different wall heat flux are presented, and thus thermal effects on the flow stability are discussed. Reynolds number and thermal effects on distributions of wall skin friction, pressure coefficient, and Nusselt number are presented and discussed.