Direct Heat Transfer Research Articles

Simultaneous integration of water and energy in heat‐integrated water allocation networks

This article proposes a new methodology for simultaneous integration of water and energy in heat‐integrated water allocation networks (WAHEN). A novel disjunctive model is first developed to determine an optimal water allocation network (WAN) where water and energy are integrated in one step. Based on the optimal WAN, a detailed heat exchanger network (HEN) to satisfy the utility target is then synthesized. Although the final network structure is obtained through two steps, the targets of freshwater and utility are optimized simultaneously. The proposed method has specific advantages. First of all, it can capture a tradeoff among freshwater usage, utility consumption, and direct heat transfer by nonisothermal mixing. Second, it can greatly reduce the complexity of subsequent HEN design. Finally, it is effective for simultaneous water and energy integration in large‐scale WAHEN systems. The advantages and applicability of this new method are illustrated by three examples from literature. © 2015 American Institute of Chemical Engineers AIChE J, 61: 2202–2214, 2015

CFD based approach for modeling direct contact condensation heat transfer in two-phase turbulent stratified flows

This paper describes a CFD based strategy for the modeling of stratified two-phase flows with heat and mass transfer across a moving steam-water interface due to direct contact condensation. Such flows have been of major importance for example in connection with the analysis of nuclear reactor safety systems, in particular during two-phase Pressurized Thermal Shock (PTS) scenarios. The approach is based on the two-fluid phase-average model. The interfacial friction was modeled by using an Algebraic Interfacial Area Density (AIAD) framework where the drag coefficient is a function of the local flow characteristics. To show the impact of the modeling of interfacial friction the simulation with the AIAD model was compared with a simulation where a constant drag coefficient of 0.44 was used in the whole domain. For the modeling of interfacial heat and mass transfer two correlations for the water heat transfer coefficient based on the penetration theory were utilized. The CFD simulations were validated against a steady-state TOPFLOW-PTS steam/water experiment. In the experiment, very detailed temperature measurements were conducted using special thermocouple lances and infrared thermography. Total condensation rate was determined indirectly by using three different methods. The simulations have shown that the results obtained with the AIAD model are considerably closer to the experimental observations than the results obtained with the constant drag coefficient. The condensation models used in the current study predict quite different total condensation rates. That caused significant differences in the temperature field. The simulations of the TOPFLOW-PTS steam/water experiment with condensation have shown that the proposed CFD modeling approach can be successfully applied for the prediction of temperature field and condensation rate during two-phase Pressurized Thermal Shock scenarios. However, the modeling of turbulent interfacial heat transfer should be improved.

Novel waste printed circuit board recycling process with molten salt

The objective of the method was to prove the concept of a novel waste PCBs recycling process which uses inert, stable molten salts as the direct heat transfer fluid and, simultaneously, uses this molten salt to separate the metal products in either liquid (solder, zinc, tin, lead, etc.) or solid (copper, gold, steel, palladium, etc.) form at the operating temperatures of 450–470°C. The PCB recovery reactor is essentially a U-shaped reactor with the molten salt providing a continuous fluid, allowing molten salt access from different depths for metal recovery. A laboratory scale batch reactor was constructed using 316L as suitable construction material. For safety reasons, the inert, stable LiCl–KCl molten salts were used as direct heat transfer fluid. Recovered materials were washed with hot water to remove residual salt before metal recovery assessment. The impact of this work was to show metal separation using molten salts in one single unit, by using this novel reactor methodology.•The reactor is a U-shaped reactor filled with a continuous liquid with a sloped bottom representing a novel reactor concept.•This method uses large PCB pieces instead of shredded PCBs as the reactor volume is 2.2L.•The treated PCBs can be removed via leg B while the process is on-going.

Experimental and numerical investigation of the effect of mixing section length on direct-contact condensation in steam jet pump

The phenomenon of direct-contact condensation is very important with respect to several industrial applications, such as steam jet pumps, direct-contact heat exchangers and nuclear reactor cooling systems. The direct and quick transfer of heat, mass and momentum across the steam–water interface makes the physics of direct-contact condensation very complex. In this work the phenomenon of direct-contact condensation, in steam jet pump, has been studied experimentally and computationally. The length of the mixing section has been varied to study its effect on the phenomenon occurring there. The experimental and CFD results obtained in this study match closely with each other and indicate towards an important process known as interface vibration process in direct-contact condensation of steam jet pump. It was found that this process has a strong dependence on the length of mixing section and plays an important role in producing high suction at the water nozzle outlet. As a result we get efficient transfer of heat, mass and momentum across the steam–water interface in steam jet pump.

Directional architecture of graphene/ceramic composites with improved thermal conduction for thermal applications

Directional heat transfer can provide an efficient way for thermal management in thermal transfer, thermal energy storage, etc. A novel growth method is proposed to synthesize continuous graphene films on insulating substrates by Ni-assisted chemical vapor deposition (CVD) at relatively low temperature down to 800 °C. Uniformly dispersed Ni nanoparticles on ceramic substrates play an important role of capturing carbon atoms and accelerating the nucleation to grow high quality graphene rooted on insulating ceramic substrates (anodic aluminum oxide, cordierite). The graphene species consist of 1D isolated graphene tubes coated on AAO, which can act as the media for directional thermal transport. The graphene/Ni/cordierite composite contains an interconnected macroporous graphene framework with a low sheet electrical resistance down to 8.6 Ω sq−1 and thermal conductivity of 4.17 W m−1 K−1. The porous graphene/Ni/cordierite composite can hold phase change materials (wood's alloy) to construct efficient thermal energy storage devices due to its high thermal conductivity, which can be used as heat sinks in thermoelectric devices. This work displays the great potential of CVD direct growth of graphene on insulating porous substrates for directional heat conduction, thermal management and thermoelectric applications.

Transient thermal analysis of a tri-axial HTS cable on fault current condition

High-temperature superconducting (HTS) tri-axial cable, which consists of three concentric phases, was developed as a potential commercial solution for next generation distribution power network. In our previous research, we simulate the transient thermal behavior of the cable by solving the heat equation using one-dimension difference method. The result shows that it takes time to recover the cable temperature to the steady-state operation level due to a low thermal conductivity of the insulation layer after a fault. However for a long cable system, when middle phase in concentric structure is rated under an over current, accumulated heat from middle phase might continually warm up the liquid nitrogen (LN2) flow by heat transfer even the over current has been stopped. In this research, we improve the numerically calculation which includes the consideration of flowing liquid nitrogen and the heat transfer in both radius and longitudinal directions. A long tri-axial cable system thermal stability is discussed based on the calculation results.

Identification of thermal boundary conditions in heat exchangers of fluidized bed boilers

A CFD simulation was carried out for the platen superheater placed in the combustion chamber of the CFB boiler. Velocity, pressure, and temperature of the steam as well as the temperature of the tube wall with the complex cross section were computed using the ANSYS/CFX software. The direct and inverse problems were solved. In the first inverse problem, the heat transfer coefficient on the flue gas side was determined based on the measured steam temperature at the inlet and outlet of the three pass steam superheater. In the second inverse problem, the inlet steam temperature and the heat transfer coefficient on the flue gas side were estimated using measured steam temperatures at selected locations of the superheater. The first inverse problem was solved iteratively using the secant method. The Levenberg-Marquardt method was used to solve the second inverse problem. At every iteration step, a direct conjugate heat transfer problem was solved using the ANSYS/CFX software.

De- and rehydration of Ca(OH)2 in a reactor with direct heat transfer for thermo-chemical heat storage. Part B: Validation of model

Heat storage technologies are used to improve energy efficiency of power plants and recovery of process heat. Storing thermal energy by reversible thermo-chemical reactions offers a promising option for high storage capacities especially at high temperatures. Due to its low material cost the use of the gas–solid reaction Ca(OH)2⇌CaO+H2O has been suggested. In Part A of this work the thermal behavior of a reactor with direct heat transfer was experimentally investigated. In this part a two-dimensional model is applied for the specified system. The experimental and simulated results during the exothermic hydration are discussed in order to confirm the validity of the model. The model is validated regarding heat transfer, integral reaction rate and maximum temperatures. In addition, an adaptation of the kinetic equation is proposed in order to take into account rate-limiting effects due to agglomeration in the powder bed.

A Fast Numerical Simulation for Modeling Simultaneous Metal Flow and Solidification in Thin Cavities Using the Lubrication Approximation

A numerical algorithm for modelling steady flow of liquid metal accompanied by solidification in a thin cavity is presented. The problem is closely related to a die cast process and in particular to the metal flow phenomenon observed in thin ventilation channels. Using the fact that the rate of metal flow in the channel is much higher than the rate of solidification, a numerical algorithm is developed by treating the metal flow as steady in a given time-step while treating the heat transfer in the thickness direction as transient. The flow in the thin cavity is treated as two dimensional after integrating the momentum and continuity equations over the thickness of the channel, while the heat transfer is modelled as a one-dimensional phenomenon in the thickness direction. The presence of a moving solid-liquid interface introduces non-linearity in the resulting set of equations, and which are solved iteratively. The location and shape of the solid-liquid interface are found as a part of the solution. The staggered grid arrangement is used to discretize the flow governing equations and the resulting set of partial differential equations is solved using the SIMPLE algorithm. The thickness direction heat-transfer problem accompanied by phase change is solved using a control volume formulation. The results are compared with the predictions of the commercial software FLOW3D® which solves the full three-dimensional set of flow and heat transfer equations accompanied with solidification. The Reynolds's lubrication equations accompanied by the through-the-thickness heat loss and solidification model can be successfully implemented to analyze flow and solidification of liquid metals in thin channel during the die cast process. The results were obtained with significant savings in CPU time.

Experimental visualization and heat transfer analysis of the oscillatory flow in thermoacoustic stacks

The oscillatory flow field induced by an acoustic standing wave is visualized at the end of the parallel-plate thermoacoustic stacks by Particle Image Velocimetry (PIV) measurements. At first, the velocity fields and vorticity maps in two typical stages of the oscillatory flow are displayed. The heat transfer processes in oscillatory flow are analyzed and discussed. Then, the influences of some key factors including the driving power, the plate thickness, the spacing and the plate shape end on the ejection vortices are presented. Finally, the displacement amplitude of suction vortices is revealed in the channel between the plates. The results indicate that the driving power, the plate thickness and the spacing should be well optimized such that more working gas can be ejected from the ends of the stacks and longer moving path of the ejection vortices can be obtained, which benefits the heat transfer between the working fluid and the heat exchanger. Also, it is shown that the displacement amplitude of the suction vortices in the channel can be enhanced with increasing the driving power. In the design of the stacks, the plate length should be longer than the displacement amplitude of the suction vortices such that the direct heat transfer between the two heat exchangers at the ends of the stack can be avoided.

De- and rehydration of Ca(OH)2 in a reactor with direct heat transfer for thermo-chemical heat storage. Part A: Experimental results

Heat storage technologies are used to improve energy efficiency of power plants and recovery of process heat. Storing thermal energy by reversible thermo-chemical reactions offers a promising option for high storage capacities especially at high temperatures. Due to its low material cost the use of the reversible reaction Ca(OH)2⇌CaO+H2O has been suggested. This paper reports on the thermal behavior of a reactor with direct heat transfer between the gaseous reactant and the solid material. Cycling stability is confirmed and the impact of the most significant parameters such as the maximum possible enthalpy difference of the heat transfer fluid between inlet and outlet, the heat transfer, the particle reaction rate and the mass transport is derived. In the test system the particle reaction rate could be identified as the main limiting parameter.

An Assessment of Direct Chip Cooling Enhancement Using Pin Fins

The use of pin fins to provide direct cooling of computer chip mounted on a printed circuit board (PCB) is studied experimentally. This concept is targeted toward improving the thermal design for the increasingly popular compact personal computer options such as the laptop notebook and web book, in which the use of a conventional heat sink might not be feasible or desirable due to the lack of space, as well as the light weight requirement. Experimental data show that the installation of a pin fin between a chip and the PCB can provide significant direct heat transfer enhancement. Data for pin heights of 4 mm, 7 mm, and 10 mm are presented. Results show that for typical chip size and power, pin fins can lead to an order of magnitude increase in the heat transfer coefficient compared to the flat plate forced convection results. While the existing heat transfer and pressure drop correlations yield good qualitative agreement with the data, the correlation tends to underpredict the heat transfer coefficient obtained in the present work.

Tailoring effective thermoelectric tensors and high-density power generation in a tubular Bi0.5Sb1.5Te3/Ni composite with cylindrical anisotropy

Transverse thermoelectric responses in heterogeneous composites made of periodically laminated Bi0.5Sb1.5Te3/Ni in a tubular shape were investigated. Numerical calculations quantitatively clarify the relationship between geometrical parameters and effective thermoelectric tensors. In the present tubular heterogeneous composites, the temperature gradient across the radial direction yields a transverse voltage along the axial direction due to the unnatural cylindrical anisotropy. The tubular configuration allows for direct and efficient heat transfer from fluid heat sources. A high-density power generation of 417 W m−2 was achieved under the small temperature difference of 83 K.

Study on the Hybrid Liquid Desiccant Evaporative Cooling Air-Conditioning Refrigeration Circulation System

This article presents a new hybrid evaporative cooling air-conditioning refrigeration system using two-stage liquid desiccant, and carries off the theoretical study on it. Compared with the conventional liquid desiccant air-conditioning refrigeration system, the new system adopts direct mixing heat transfer technology and evaporative cooling technology instead of water-cooled or air-cooled technology. Thus the cooling water usage is reduced. Using the two-stage adiabatic absorber and using refrigerant rather than water as the cooling medium of desiccant solution, the pressure difference of mass transfer is increased. Compared with the conventional system, the heat of regenerating solution is decreased because the mass ratio of air to desiccant solution is increased which gives rise to the decline of solution's mass rate. The results of theoretical calculation show that the new system has the better performance in power consumption, usage of water and equivalent coefficient of performance than the conventional system.

A review of fire blocking technologies for soft furnishings

AbstractFire barrier fabrics are expected to play an increasingly important role in complying with existing and proposed soft furnishing flammability regulations in the US. The number of commercial fire blocking technologies is large in order to accommodate the vast requirements of the consumers, manufacturers, and regulatory agencies. Generally, highloft, nonwoven fiber battings are used in residential mattress applications, whereas coated or laminated textiles are more common in institutional and upholstered furnishing applications. Successfully achieving the desired level of fire protection requires appropriate matching of the barrier fabric to the desired characteristics of the soft furnishing. Barrier material selection for soft furnishings is generally a process of trial and error due to significant measurement science gaps.In 2009, the National Institute of Science and Technology and American Fiber Manufacturers Association held a workshop on fire blocking barrier fabrics for soft furnishings to discuss the past, present, and future state of the barrier materials in the US. This manuscript is based on knowledge obtained from the workshop and the subsequent knowledge gathered from literature and stakeholders. Several fire blocking technologies have been explored to reduce the flammability of soft furnishings by preventing or delaying direct flame impingement and heat transfer from the flames or molten polymer to the core components. While previous studies reported on use of fire barriers to comply with full-scale testing of soft furnishing items, they failed to report on assessment of barrier materials as isolated components. In addition to a few examples that demonstrate the complexity that makes a priori selection of fire barrier materials difficult, various fire blocking technologies are discussed in this report with respect to material type, fiber content, and fire blocking mechanisms. Potential test methods for characterizing barrier performance are reviewed. Future trends in fire blocking materials are also briefly described.

Geocooling potential of borehole heat exchangers' systems applied to low energy office buildings

Abstract The potentiality of geocooling (i.e. free cooling) with borehole heat exchangers is analysed for low energy office buildings. The borehole heat exchanger field is coupled to a heat pump in winter and to the cold distribution system in summer through a flat plate heat exchanger. The cooling requirement satisfied by a direct heat transfer into the ground through the borehole heat exchangers is so-called geocooling. A dynamic system model has been developed to simulate the building, the emission of thermal energy through thermally activated building systems, the technical installation including the borehole heat exchanger field and the interconnected thermal interactions. Thermal comfort requirements determine the building energy needs and the size of the ground coupled system. A methodology is presented for best system design. Building design, system technical feasibility and limits of the ground coupled system are discussed and result from the analysis of the numerous system simulations. Geocooling potential depends on the quality of the building design and its heat emission. The importance of the ground thermal conductivity and the ground recharge ratio on the system design are highlighted. Simple design sizing keys are proposed for a fast pre-sizing of a borehole field.

Process integration between individual plants at a large dairy factory by the application of heat recovery loops and transient stream analysis

Abstract Indirect heat transfer between individual plants at a large industrial site, such as at a large dairy factory, can be accomplished using a Heat Recovery Loop (HRL) operating at relatively low temperatures (20–50 °C). Direct heat transfer between plants is problematic because of the production schedule and frequent process start-ups and shutdowns that occur due to the need for regular cleaning. The dynamic operation of an industrial HRL at a large dairy factory is presented and briefly discussed. A model HRL based on a set of variable industrial stream data is used to examine the effect of heat storage capacity and HRL temperature range on the amount of heat recovery. Both HRL temperature range and a small heat storage volume can significantly improve the amount of heat recovery. The interface level in the storage tank is also modelled for different volumes and the resulting system is very dynamic and therefore good control needs to be incorporated into the HRL.

Simultaneous heat exchanger network synthesis for direct and indirect heat transfer inside and between processes

Process plants are typically divided into different process parts having specific processing tasks with possibly different ownership. Heat integration between these processes can increase the energy- and economic efficiency for both the overall plant and the individual processes. In this paper we present a heat exchanger network synthesis MINLP-model that allows simultaneous heat integration directly between streams in the same process and both directly and indirectly between streams in different processes. The indirect heat transfer is accomplished by using intermediate streams. Two examples, one small explanatory one and one from the literature, are optimized. The results verify that the model works logically.

Experimental Analysis of Seed Effect on the Directional Solidification of Sn-Pb Alloy

In this paper, an experiment model for the directional solidification of Lead/Tin alloy is built and the effects of different-shape seeds on the microstructures on the solidification microstructure are investigated. In a casting process, the temperature and concentration fields will affect the microstructures of materials and this influence is the key point of improving their mechanical and physical properties. It is not easy to control the morphology of solidifying microstructures. The scheme of directional solidification can make the microstructures grow along a fixed direction and it is also the base of single-crystal growth. In the experiment, a poly-grain seed with the same initial concentration of the solidifying casting is used to induce the columnar growth at the bottom portion of the casting, which could avoid the equiaxed growth due to the high undercooling or cooling rate there. In the experimental analysis, we studied the influences of different geometry seeds on the constrained growth, the preferential growth direction of dendrite, the grain size, the temperature gradient, the growth rate, the primary arm spacing and the secondary arm spacing. From the microstructure observation, the adding seed casting reduced the chill-affected and extended the directional solidification zone. This is expected to have the better or more complete structure of directional solidification. Keywords: Directional Solidification, Seed, Heat Transfer and Microstructure

The Effect of H<sub>2</sub>S on Hydrogen and Carbon Black Production from Sour Natural Gas

Hydrogen and carbon black production via thermal decomposition of natural gas have been achieved using a carbon black furnace. Direct heat transfer from inert hot gases (argon) introduced into sour gas. The carbon black furnace is a small-scale axial flow reactor. There are 3 main chemical reactions through which carbon black is produced. Finally results indicate that the most influencing factor over these chemical reactions and CH4 and H2S conversions is reactor temperature.