Ruina Xu

• PhD
• Professor (Full) at Tsinghua University

Geological carbon sequestration (GCS) in deep saline aquifers is an effective means for storing carbon dioxide to address global climate change. As the time after injection increases, the safety of storage increases as the CO2 transforms from a separate phase to CO2(aq) and HCO3- by dissolution and then to carbonates by mineral dissolution. However...

Using carbon dioxide as a displacing fluid to enhance shale gas recovery is a promising technique given its potential for significant contributions to both unconventional resource development and CO2 geological sequestration. The adsorption capacity of CO2 in nanoscale shale organic pores is the key issue to evaluate the feasibility of CO2-enhanced...

With the urgent demands of ultra-high heat flux cooling technology, enhancement of spray cooling by surface structure as an effective thermal management method has drawn increasing attention. Combining micro- and nano-structures is a promising way to improve spray cooling but few attempts have been made, especially in an application oriented closed...

Flow boiling is an important heat dissipation method for cooling high heat flux surfaces in many industrial applications. The heat transfer can be further enhanced by using porous media surfaces due to their high specific surface areas. However, although flow boiling in channels is well understood, the phase-change behavior with the additional capi...

Extreme thermal conditions with heat flux densities exceeding 1 MW m⁻² or temperatures reaching up to 1000 °C are prevalent in various situations. However, thermal protection ability depends on specialized materials or is currently limited with existing cooling schemes. Herein, an innovative cooling scheme that relies on evaporation‐driven capillar...

Thermal-hydraulic coupling system, a physical process where flow and heat transfer interact mutually, is ubiquitous in nature and industrial applications. Although experimental measurements and solution of governing equations can be used to quantify physical fields such as velocity and temperature, acquiring dense and high-fidelity data is still ch...

Deep space exploration expands our understanding about the evolution history of solar system, while the future development heavily relies on the construction of energy systems and utilization of resources on the planet. This paper systematically reviewed the progress in the environmental control and construction technologies of space bases, extrate...

The issue of salt precipitation during CO2 geological storage in saline aquifers has presented significant challenges to the injectivity of current CO2 geological storage projects. Numerous research efforts have been dedicated to investigating this problem; however, many laboratory studies have overlooked the influence of boundary conditions on sol...

The purpose of this research was to provide further understanding of turbulent dynamics and heat transfer mechanisms in accelerating flows with thermophysical variations and pressure drops in micron tubes. Direct numerical simulations were conducted to investigate the turbulence to supercritical pressure ${\rm CO}_2$ in heated micron tubes with inn...

The requirements for new generation vehicles in terms of the flight speed, thrust–weight ratio, and maneuverability necessitate the development of high performance and reliable propulsion systems where active thermal protection technology plays a crucial role. Transpiration cooling based on a microporous structure is considered as one of the most p...

Solving flow problems based on the Reynolds-averaged Navier-Stokes equations is a dominant method in terms of efficiency and accuracy for design and analysis of engineering applications, while the performance of RANS models in predicting the turbulent heat transfer of supercritical pressure fluid could be severely poor. To a great extent, the model...

Hydraulic fracturing is widely used in shale gas development, while it could cause decline in production in water-sensitive formations. Using CO2 for reservoir fracturing, injection to enhance gas recovery, and sequestration in depleted shale reservoirs can solve this problem, increase shale gas production, and store CO2 underground. Given the high...

This work aimed to study the intensification of flow boiling heat transfer and critical heat flux (CHF) under conditions of highly reduced pressures due to a modification of the inner wall surface of a mini-channel. Such research is relevant to the growing need of high-tech industries in the development of compact and energy-efficient heat exchange...

The supercritical CO2 (sCO2) Brayton cycle has the advantages of high efficiency, good flexibility and compact equipment, and is widely regarded as the ideal power cycle for the new generation concentrating solar power (CSP). The application scenario of the CSP determines that the unit's fast peak shaving capability must be considered. In this pape...

CO 2 capture is a process with a high energy consumption, and its large-scale implementation should be based on comprehensive analysis of its impact on the energy, economy, and environment. The process of injecting CO 2 into existing oil fields is a well-known enhanced oil recovery (CO 2 -EOR) technique. Using CO 2 as a working fluid to recover oil...

Injectivity is an important indicator when it comes to evaluating the geological storage of CO2. It is commonly accepted that salt precipitation damages the injectivity during the CO2 injection process. Numerous studies have been conducted on the salt precipitation mechanism in homogeneous reservoirs. However, the mechanism of salt precipitation in...

Shale oil has been receiving more and more attention, because of its promising contribution to energy development and utilization. Capillary imbibition in shale nanopores widely exists in the storage and recovery of shale oil. Despite the ubiquitous nature, shale oil imbibition in nanoconfined space and the effect of nanopore walls on flow are stil...

Carbon capture, utilization and storage (CCUS) refers to the separation of CO2 from energy utilization systems, industrial production or the atmosphere followed by purification and transport to facilities using CO2 or to storage sites to achieve long-term separation of the CO2 from the atmosphere. The Intergovernmental Panel on Climate Change (IPCC...

CO2 enhanced shale gas recovery is considered to be a potential method of shale gas development and CO2 reduction. However, core experiments cannot independently analysis competitive adsorption when the partial pressure of CH4 in the core sample keeps changing during displacement. In this study, an experimental method based on low-field nuclear mag...

Porous media are used widely in technological (e.g., transpiration cooling and heat sinks for electronic devices), and biological (e.g., bone tissue) applications. The requirement for performance optimization demands a transition from passive selection to actively designing the topology of porous media. Triply periodic minimal surface (TPMS) struct...

Supercritical pressure fluids are widely used in heat transfer and energy systems. The benefit of high heat transfer performance and the successful avoidance of phase change from the use of supercritical pressure fluids are well-known, but the complex behaviours of such fluids owing to dramatic thermal property variations pose strong challenges to...

CO2 huff-n-puff is one of the most important methods to enhanced oil recovery, which can simultaneously improve oil production and achieve CO2 geological storage. Due to the fluctuation of formation pressure, water-oil-CO2 three phase migration in porous structures was affected significantly by supercritical CO2 exsolution and expansion. However, t...

Shale gas is an important unconventional gas resource because of its rich storage ability. The quantitative measurement of adsorption capacity is essential to shale gas exploration. However, traditional measurements, such as gravimetric and volumetric method, are influenced by excess adsorption effect and need to be corrected at high pressures. In...

Currently, there are no universal methods for calculating the heat transfer and pressure drop for a wide range of two-phase flow parameters in mini-channels due to changes in the void fraction and flow regime. Many experimental studies have been carried out, and narrow-range calculation methods have been developed. With increasing pressure, it beco...

The accurate evaluation of gas content is significant for the efficient exploration of unconventional natural gas reservoir, direct method adopts lost gas model and combines with desorption curve to evaluate reservoir gas content. However, the classical lost gas model is derived from constant pressure condition and spherical particle assumption in...

The rapid development of electronics, energy and propulsion systems has led us to the point where their performances are limited by cooling capacities. Heat fluxes of 10~100, even over 1,000 W/cm2 need to be dissipated with minimum coolant flow rate in next-generation power electronics. Spray cooling is a high heat flux, uniform and efficient cooli...

Two-phase jet impingement cooling with carbon dioxide at subcritical pressures of 6.5 and 7.0 MPa (near-critical pressure) on smooth surface was experimentally studied in this paper. Local heat transfer coefficients were measured with a homemade integrated thermal chip. The effects of heat flux, pressure and mass flow rate on heat transfer were stu...

Extracting heat from hot dry rock by injecting working fluids is recognized as an important way of geothermal utilization. In recent years, using CO2 as the working fluid has been paid more attentions since its advantage of saving water, appropriate properties, as well as the potential capacity of large-scale CO2 geological storage. Our previous re...

Experimental and numerical analyses on the thermal and hydrodynamic performance of volumetric solar receivers were conducted. Silicon carbide (SiC) absorbers with various pore structures were systematically investigated on a laboratory-scale test platform. A three-dimensional theoretical model coupling the fluid flow with internal heat transfer was...

The inclined pipe is widely used as a downward particle-conveying structure. Bubbles accompanying with the descending particles always induce complex flow pattern and pipe vibration, which would shorten the service life of equipment. In this paper, the gas-solid interaction and fluid-vibration coupling were experimentally studied by analyzing dynam...

Porous structures are ubiquitous in many thermal management and energy conversion systems. The morphology of a porous structure has significant impact on the fluid flow, heat/mass transport, and strength performance. However, the available fabrication techniques are not capable of directly tailoring porous structures with well-controlled pore featu...

The vicinity of the droplet three-phase contact line can be divided into four regions depending on the dominant forces and the liquid film thickness: the absorbed film region, the transition region, the intrinsic meniscus region, and the microconvection region, wherein the transition region has the largest evaporation rate for smaller thermal resis...

A pore-scale high-pressure visualization experimental system is used to investigate CO 2 exsolution during the CO 2 huff-n-puff process for enhanced oil recovery and geological CO 2 storage. Eighteen different experimental cases are examined to investigate the mechanisms by which depressurization-induced CO 2 exsolution is affected by a near-miscib...

A pore-scale high-pressure visualization experimental system is used to investigate CO2 exsolution during the CO2 huff-n-puff process for enhanced oil recovery and geological CO2 storage. Eighteen different experimental cases are examined to investigate the mechanisms by which depressurization-induced CO2 exsolution is affected by a near-miscible v...

A pore-scale high-pressure visualization experimental system is used to investigate CO2 exsolution during the CO2 huff-n-puff process for enhanced oil recovery and geological CO2 storage. Eighteen different experimental cases are examined to investigate the mechanisms by which depressurization-induced CO2 exsolution is affected by a near-miscible v...

A pore-scale high-pressure visualization experimental system is used to investigate CO2 exsolution during the CO2 huff-n-puff process for enhanced oil recovery and geological CO2 storage. Eighteen different experimental cases are examined to investigate the mechanisms by which depressurization-induced CO2 exsolution is affected by a near-miscible v...

With the rapid development of thermal management technology, flow boiling in porous media has been widely used in many applications. Among them, phase-changed transpiration cooling is a potential trend in the field of aerospace thermal protection. However, the dynamic response delay, change in boiling point at low pressure, and vapor blockage effec...

The flow boiling in microgap is attracting much attention due to the large potential for directly cooling the heat sources of the electronic devices, which has many advantages like high heat dissipation rate, easy fabrication, and less working fluid consumption while supplying a large cooling area, etc. However, the local dryout easily occurs near...

Injectivity is one of the most important factors to evaluate the feasibility of CO2 geological storage. Salt precipitation due to the mass of dry CO2 injected into the saline reservoir may cause a significant decrease in injectivity. However, the coupling effect of injection parameters and reservoir conditions on salt precipitation are not clear. H...

Convection heat transfer of supercritical pressure CO2 in various channels has extensive applications in many industries. In some applications, the channel may not be straight, but rather serpentine. Researchers have found that the transition to turbulence of constant-property fluid is delayed in a constant-curvature pipe, whereas it is promoted in...

Self-pumping transpiration cooling is an advanced and effective method for cooling a high heat flux surface without the assistance of pumps and control systems. This study demonstrates a novel structure for self-pumping transpiration cooling with a protective porous armor structure. The results showed that the cooling system successfully drove liqu...

The open microchannel configuration can mitigate several drawbacks of closed microchannel for flow boiling heat transfer, but the studies related to flow boiling in open microchannels are limited up to now. In this study, experiments were conducted to investigate the heat transfer and pressure drop characteristics during flow boiling of deionized w...

This study experimentally investigated self-pumping transpiration cooling in a hot wind tunnel. The coolant (liquid water) automatically and continuously flowed from the water tank to the heated porous surface without the use of any pump and then evaporated on the porous surface, thereby dissipating a large amount of heat. The self-pumping cooling...

Nanoporous materials have a wide range of applications in clean energy and environmental research. The permeability of nanoporous materials is low, which affects the fluid transport behavior inside the nanopores and thus also affects the performance of technologies based on such materials. For example, during the development of shale gas resources,...

The performance of the turbulent flow model for predicting the buoyancy effect on convection heat transfer of supercritical fluid is severely affected by strongly varying thermal physical properties near the pseudo-critical point. Over-prediction is attributed, at least partly, to the misuse of the constantly turbulent Prandtl number for the turbul...

In this paper, we conducted experimental and theoretical studies for water flash evaporation. We presented a typical flashing experimental system as well as a method for dynamic measurement of flashing mass. On the basis of the experimental results, we developed a one-dimensional mathematical model of flash evaporation as well as a numerical soluti...

Self-pumping transpiration cooling is an effective method to cool a heated surface without a pump and control system. A porous module with a tree-like micro-channel was manufactured using a metal additive manufacturing method. A metal ceramics heater was used to heat the additive manufactured porous module to test the transpiration cooling effect....

The heat transfer can deteriorate with supercritical pressure fluids flowing in vertical tubes due to buoyancy. This study used a helical insert in the tube to change the flow structure and improve fluid heat transfer. Convection heat transfer of supercritical pressure CO 2 in a vertical bare tube and with a helical insert was investigated experime...

During the geological carbon sequestration, the short-term behavior of CO2 is controlled mainly by two-phase flow of supercritical carbon dioxide (ScCO2) and water in saline aquifers. A better understanding of ScCO2-water two-phase flow in the porous media under the geological conditions will improve predictions of the long-term CO2 storage reliabi...

Channel scales vary from several centimeters to several micrometers in various industrial applications that conduct convective heat transfer at supercritical pressures. The heat transfer performance reveals relatively different features even under similar Reynolds number and thermphysical property conditions. The authors investigated the influence...

Previous study showed that hybrid micro and nano-structured surfaces could enhance spray cooling heat transfer significantly, while the enhancement mechanisms remain unclear. Here, droplet spreading and evaporation behaviors on hybrid micro and nano-structured surfaces with different surface temperatures were studied by experiments. Results showed...

Hypersonic vehicles with turbojet, ramjet, and scramjet engines are expected to be widely applied to future transportation systems. Due to high-speed flight in the atmosphere, body outer surfaces suffer strong aerodynamic heating, and on the other hand, combustion chamber inter walls are under extremely high temperature and heat flux. Therefore, mo...

Jet impingement cooling with carbon dioxide at supercritical pressures on micro pin fin structured surfaces was studied experimentally to enhance the impingement cooling heat transfer. The effects of the microstructure, mass flow rate and heat flux on the heat transfer were studied using the MEMS (Micro-Electro-Mechanical System) based silicon chip...

Enhancing spray cooling with surface structures is a common, effective approach for high heat flux thermal management to guarantee the reliability of many high-power, high-speed electronics and to improve the efficiency of new energy systems. However, the fundamental heat transfer enhancement mechanisms are not well understood especially for nanost...

Jet impingement cooling is widely used due to its high heat transfer rates, especially within the stagnation region. Jet impingement cooling by supercritical pressure fluids has even higher heat transfer coefficients with the proper working conditions than regular fluids with no burnout. This paper presents a numerical investigation of supercritica...

Turbulent convection heat transfer of fluids at supercritical pressures can experience abnormal heat transfer phenomena as the fluid temperature approaches the pseudo-critical temperature. A better understanding of these abnormal heat transfer characteristics is essential to develop ways to assure the security and reliability of engineering designs...

Spray cooling is an efficient method of heat exchange which has prominent prospects for development. Its main advantages are the high cooling capacity of unit mass fluid, the low flow rate in the system, the uniformity of temperature distribution on the surface and the strong stability of system. A closed-loop spray cooling system based on refriger...

The present work developed a three-equation local thermal non-equilibrium (LTNE) model to model the initial effect on the internal thermal conduction in the solid phase of porous medium. The two energy equations in the original LTNE model were extended with a third equation to govern the dimensionless thermal penetration depth for the internal ther...

Jet impingement cooling is widely used in many industrial applications due to its high heat transfer capability and is an option for advanced high power density systems. Jet impingement cooling with supercritical pressure fluids could have much larger heat transfer rates combining with the large fluid specific heat near the pseudocritical point. Ho...

Water flow boiling in large aspect ratio microchannels was experimentally investigated in a rectangular microchannel 300 μm deep and 6 mm wide; hence, the hydraulic diameter was 571 μm and the aspect ratio was 20. The tests used mass fluxes of G = 261–961 kg/(m² s) and heat fluxes of q″ = 631–987 kW/m² with the combined effects on the flow boiling...

Transpiration cooling is an effective way to protect high heat flux walls. However, the pumps for the transpiration cooling system make the system more complex and increase the load, which is a huge challenge for practical applications. A biomimetic self-pumping transpiration cooling system was developed inspired by the process of trees transpirati...

Closed-loop supercritical CO2 (s-CO2)Brayton cycles have advantages of higher cycle efficiency, smaller weight and volume, and less complex. The realization of higher cycle efficiency depends on a perfect thermal match in heat exchanger to fully recover heat energy from the exhausted gases from turbine. The key to increase the temperature glide mat...

CO2 sequestration in saline aquifers is a promising way to address climate change. However, the pressure of the sequestration reservoir may decrease in practice, which induces CO2 exsolution and expansion in the reservoir. In this study, we conducted a core-scale experimental investigation on the depressurization of CO2-containing sandstone using N...

Convection heat transfer of supercritical pressure fluid is important in industrial applications such as supercritical power stations, the supercritical CO2 Brayton cycle, Carbon Capture Utilization and Storage, and the thermal protection for rocket thrusters. Previous research has confirmed that there are three heat transfer regions for convection...

Flow boiling in microchannels is favored by the heat transfer community due to the high heat transfer rates that can be obtained with lower mass flow rates. However, the heat transfer rates for flow boiling in microchannels are impacted by flow reversals and flow instabilities. An open microchannel structure was recently proposed to reduce the impa...

Phase-change heat transfer on nanostructured surfaces is an efficient cooling method for high heat flux devices due to its superior wettability. Liquid droplet spreading and wicking effect then dominate the heat transfer. Therefore, this study investigates the flow behavior after a droplet touches a nanostructured surface focusing on the ZnO nanowi...

The pore-scale behavior of the exsolved CO2 phase during the depressurization process in CO2 geological storage was investigated. The reservoir pressure decreases when the injection stops or when a leaking event or fluid extraction occurs. The exsolution characteristics of CO2 affect the migration and fate of CO2 in the storage site significantly....

Confined round jet impingement cooling of a flat plate at constant heat flux with carbon dioxide at supercritical pressures was investigated numerically. The pressure ranged from 7.8 to 10.0 MPa, which is greater than the critical pressure of carbon dioxide, 7.38 MPa. The inlet temperature varied from 270 to 320 K and the heat flux ranged from 0.6...

Enhanced geothermal systems create fractured reservoirs to extract economic quantities of heat from low-permeability and/or low-porosity geothermal resources. Convective heat transfer characteristics of fluids at supercritical pressures in rock fractures are important for optimizing the heat transfer model, which is a key tool for simulating heat e...

The depletion of conventional energy sources is leading to increased demand for unconventional energy sources. Shale gas is an important unconventional energy source that has been successfully developed and utilized in the United States. CO2 enhanced shale gas recovery (CO2-ESG) is a promising shale gas extraction technology. Supercritical CO2 inje...