Heat Exchange Mechanism Research Articles

Change of the Global Ocean Vertical Heat Transport over 1993–2010

A dynamically and data-consistent ocean state estimate during 1993–2010 is analyzed for bidecadal changes in the mechanisms of heat exchange between the upper and lower oceans. Many patterns of change are consistent with prior studies. However, at various levels above 1800 m the global integral of the change in ocean vertical heat flux involves the summation of positive and negative regional contributions and is not statistically significant. The nonsignificance of change in the global ocean vertical heat transport from an ocean state estimate that provides global coverage and regular sampling, spatially and temporally, raises the question of whether an adequate observational database exists to assess changes in the upper ocean heat content over the past few decades. Also, whereas the advective term largely determines the spatial pattern of the change in ocean vertical heat flux, its global integral is not significantly different from zero. In contrast, the diffusive term, although regionally weak except in high-latitude oceans, produces a statistically significant extra downward heat flux during the 2000s. This result suggests that besides ocean advection, ocean mixing processes, including isopycnal and diapycnal as well as convective mixing, are important for the decadal variation of the heat exchange between upper and deep oceans as well. Furthermore, the analyses herein indicate that focusing on any particular region in explaining changes of the global ocean heat content is misleading.

Heat exchange studies on coconut oil cells as thermal energy storage for room thermal conditioning

As reported by many thermal environment experts, room air conditioning might be controlled by thermal mass system. In this paper we discuss the performance of coconut oil cells as room thermal energy storage. The heat exchange mechanism of coconut oil (CO) which is one of potential organic Phase Change Material (PCM) is studied based on the results of temperature measurements in the perimeter and core parts of cells. We found that the heat exchange performance, i.e. heat absorption and heat release processes of CO cells are dominated by heat conduction in the sensible solid from the higher temperature perimeter part to the lower temperature core part and heat convection during the solid-liquid phase transition and sensible liquid phase. The capability of heat absorption as measured by the reduction of air temperature is not influenced by CO cell size. Besides that, the application of CO as the thermal mass has to be accompanied by air circulation to get the cool sensation of the room’s occupants.

Ex situ calibration technique for simultaneous velocity and temperature measurements inside water droplets using temperature-sensitive particles

Phosphorescence-based temperature measurements usually employ in situ calibration in macro-scale flow domains. However, the application of conventional in situ calibration in millimetre scales such as in 2 mm droplets is difficult because temperature probing using an intrusive technique such as thermocouple-based measurement can deform a droplet interface and even its integrity. Therefore, in this paper we propose an ex situ calibration technique for combined 2D fluid temperature and flow velocity measurements inside pendant droplets using thermographic phosphorescence and particle image velocimetry. This calibration technique has the potential to perform measurements not only inside droplets but also in other small-scale fluid domains that are sensitive to intrusive temperature probing. To develop this technique, the effect of local phosphorescence intensity on the phosphorescence decay constant (initial intensity effect) was studied and quantified. We observed that the phosphorescence light intensity of the first image among the decay image series affects the local decay constants as the coefficient of a half sigmoid function. This novel ex situ calibration technique was used to investigate high-spatial-resolution temperature and flow velocity distributions inside a pendant water droplet located in an air stream, and an anti-distortion algorithm was used to correct the optical distortion induced by the curved surface of the droplet. The measured temperature and velocity fields were found to be reasonable and consistent with those obtained in previous studies. The proposed technique is expected to be useful in conducting further investigations on the mechanisms of heat and mass transfers between droplets and carrier gases. Furthermore, it can potentially improve our understanding of the heat and mass exchange mechanisms within a droplet’s internal flow structures and temperature gradients.

Control‐Oriented Modeling of Non‐Adiabatic Solid Oxide Fuel Cell Stacks

AbstractThe paper presents a lumped zero dimensional (0D) model for the simulation of solid oxide fuel cell (SOFC) stack thermal dynamics. Particularly, a dedicated sub‐model is developed, via a mixed gray/black‐box modeling approach, to suitably treat the heat exchange mechanisms between the stack and its surrounding environment. Such a sub‐model was embedded into an existing 0D simulator and then tested via comparison with temperature sensor measurements. The hardware sensor is indeed particularly critical, due to its location and the typically high operating temperature characterizing SOFC. Therefore, suitable experimental transients were performed at Topsoe fuel cell test‐bench, thus allowing verifying the accuracy and generalization granted by the 0D model in reproducing stack outlet temperature trajectories, both in steady‐state and dynamic operations. As a result, the modeling approach here proposed is proven effective either to replace the hardware temperature sensor at stack outlet or for redundancy purposes to improve reliability. Moreover, the 0D model can be deployed for offline design of SOFC systems' control strategies, as well as for real‐time diagnostic and control applications.

Rotary reverse flow reactor vs. adiabatic reactor with regenerative preheating - Design and comparison

The autothermal catalytic-combustion systems are commonly used for the purification of waste air streams contaminated with low concentrations of volatile organic compounds (VOC). Within this type of devices, the reverse flow reactors (RFR) are known to be more efficient than systems employing recuperative (surface) heat exchangers to preheat the waste air stream with the lean air effluent from the catalytic incinerator. The advantage of the RFR is basically due to the regenerative heat-exchange mechanism, provided by the inert and catalytic solids inside the unit.As an alternative, the regenerative mechanism of preheating can be achieved by an independent heat exchanger, which coupled to a catalytic reactor could be expected to produce similar performance as the RFR.In this context, this contribution is devoted to analyse comparatively the performances of a rotary reverse flow reactor (RRFR) and a system comprising a rotary regenerative heat-exchanger and a catalytic reactor (RHE-SR system) for the treatment of a waste air stream contaminated with ethanol and ethyl acetate, by means of mathematical simulation. Both alternatives are assumed to be composed of monoliths with square channels. A strategy of design for both systems suitable for their comparison is proposed, attending to a range of VOC concentration in the waste stream. Both alternatives can be regarded as being suitable options to carry out the target. However, the resulting designs show clear advantages in favour to the RRFR, as this alternative requires a significantly more compact equipment than the RHE-SR does and, besides, it allows to be operated under a wider range of the rotational speed, which is the main control variable once the systems are operating.

Influence of Thermal Diffusion on the Hysteresis Area of Hydrogen Impurities’ Catalytic Combustion on Platinum Wire

A complex investigation of the hysteresis mechanisms of heat and mass exchange and catalytic flameless combustion of hydrogen-air mixtures on platinum’s particles and wires. Thermal diffusion of hydrogen does not affect the critical conditions for self-ignition mixture unlike conditions extinction mixture. Method is proposed the analytical determination of the hysteresis areas heat and mass exchange particles (wires) catalyst. Inside the areas there is a catalytic ignition, and outside - stationary states of oxidation and catalytic combustion of combustible gas in the air.

Influence of the Local Cross-Streamlined Obstacles on the Velocity and Pressure Fluctuations

Background. Developing means to enhance the effectiveness of mass and heat transfer processes on streamlined surfaces and their hydroacoustic qualities in the objects of power, aerospace and marine engineering. Objective. The purpose of the research is to determine the formation features and space-time characteristics of the velocity and wall-pressure fluctuation fields in flows above and on the streamlined surfaces with local obstacles as the cross-streamlined half-cylindrical trench on a flat surface and circular obstacle on the flexible longitudinal hydroacoustic antenna surface. Methods. The research was carried out by means of hot-wire and hydroacoustic measuring in flows on streamlined obstacles and spectral analysis of statistical characteristics of the velocity and wall-pressure fluctuations in investigating systems. Results. It has been determined experimentally the zones and conditions of vortices formation by various frequency ranges, the extremes of power spectrum densities of the velocity and wall-pressure fluctuations, vortex interaction, ejections and stalls near streamlined surfaces with local obstacles at rather wide Reynolds number ranges. Conclusions. Basing on received space-time characteristics of the velocity and wall-pressure fluctuations and discovered typical features of their spectra the hydromechanical mechanisms of heat and mass exchange and hydroacoustic processes in the investigated systems have been determined. The results of the study can be used for designing of such systems and they reveal the perspective of directed control by mentioned processes for increase of their efficiency.

Boiling in volume of low temperature droplets of dispersed phase of liquid emulsions

Mechanisms of droplets boiling in dispersed liquid-liquid systems differ significantly from more commonly studied and better understood mechanisms of vaporization in homogeneous fluids. For the analysis of boiling of liquid emulsions with low-temperature dispersed phase the theory of Labuntsov has been used, which was developed for boiling of homogeneous fluid near a solid wall. Model representations of more complex physical phenomena were substantiated using the similarity criteria, corresponding to the character of the heat exchange mechanisms with the dispersed droplets in the analogous conditions. A modified calculated dependence has been obtained for liquid emulsions with the low boiling dispersed phase. The results of the calculations are compared with the data of visual observations and experimental studies from the literature. It turned out that depending on the concentration of dis-persed phase heat transfer in emulsions is characterized by two regimes with different nucleate boiling mechanisms. Good agreement was indicated with measured data of heat flux at boiling of emulsions of water-in-oil type.

Investigation of the thermal properties of spacer fabrics with bio-ceramic additives using the finite element model and experiment

The heat resistance of fabric enhanced by bio-ceramic additives (BCAs) is investigated theoretically and experimentally in order to determine the influence of modification of the infrared (IR) absorption property of the fabric. The enhanced IR sensitivity of textiles improves the thermoregulatory processes when worn in cold environments. The finite element model has been developed by taking into account the coupled phenomena of heat conduction, surface convection and the interaction of the fabric with IR power flux by employing heat transfer differential equations and the Stefan–Boltzmann law. Evaluations of IR absorptivity, reflectivity and transmissivity, the temperature transients during the hot plate chamber test and heat retaining properties of the fabric heated by an IR lamp have been obtained experimentally and simulated by means of the developed finite element model. The values of model parameters have been found, which provided a satisfactory match between the computation and the experiment in all considered cases. Simultaneously, the obtained values were reasonably close to rough theoretical estimations. Efforts have been made to distinguish from each other the influence of diffusive and radiative components of heat transfer, which affect the results of thermal resistance tests. The comparative analysis of contributions of different heat exchange mechanisms allows a better understanding of the peculiarities of standard heat resistance measurement procedures applied to BCA-enhanced fabrics and facilitates the validation of the computational models.

Thermodynamical analysis of a quantum heat engine based on harmonic oscillators.

Many models of heat engines have been studied with the tools of finite-time thermodynamics and an ensemble of independent quantum systems as the working fluid. Because of their convenient analytical properties, harmonic oscillators are the most frequently used example of a quantum system. We analyze different thermodynamical aspects with the final aim of the optimization of the performance of the engine in terms of the mechanical power provided during a finite-time Otto cycle. The heat exchange mechanism between the working fluid and the thermal reservoirs is provided by the Lindblad formalism. We describe an analytical method to find the limit cycle and give conditions for a stable limit cycle to exist. We explore the power production landscape as the duration of the four branches of the cycle are varied for short times, intermediate times, and special frictionless times. For short times we find a periodic structure with atolls of purely dissipative operation surrounding islands of divergent behavior where, rather than tending to a limit cycle, the working fluid accumulates more and more energy. For frictionless times the periodic structure is gone and we come very close to the global optimal operation. The global optimum is found and interestingly comes with a particular value of the cycle time.

Study of HTS Machine System Cooling With a Closed-Loop Thermosyphon: Stability of Unsteady Heat Load and Transient Conduction

Thermosyphoning (TS) is a heat exchange mechanism that uses a circulating refrigerant based on natural convection with no need for a mechanical pump. This cooling technique has been employed to circulate liquid coolant and gaseous coolant for high-temperature superconductor (HTS) rotating machine systems. These machines operate with an HTS field pole in the temperature range from 25 to 40 K, which are suitable temperatures for the survivability of HTS wires. The cryomechanical components are simple; there are a condenser, an evaporator, and adiabatic tubes located between the condenser and the evaporator. For applications to ship propulsion motors or generators of the radial gap type, the liquid/gas coolant flow along the adiabatic tube inside the rotating shaft may be considerably affected by the ship's rolling and/or pitching motion. In this work, we studied the temperature stability and heat load survivability of cryogenic TS, by constructing a 100 W @ 30 K-grade model. We investigated the temperature, thermal resistance, and heat flux, with a maximum load up to 60 W, of ground and onboard tests. The observed TS behavior indicates that there is sufficient survivability to assemble megawatt-class ship propulsion systems.

Weak Jets and Strong Cyclones: Shallow-Water Modeling of Giant Planet Polar Caps

Abstract Giant planet tropospheres lack a solid, frictional bottom boundary. The troposphere instead smoothly transitions to a denser fluid interior below. However, Saturn exhibits a hot, symmetric cyclone centered directly on each pole, bearing many similarities to terrestrial hurricanes. Transient cyclonic features are observed at Neptune’s South Pole as well. The wind-induced surface heat exchange mechanism for tropical cyclones on Earth requires energy flux from a surface, so another mechanism must be responsible for the polar accumulation of cyclonic vorticity on giant planets. Here it is argued that the vortical hot tower mechanism, claimed by Montgomery et al. and others to be essential for tropical cyclone formation, is the key ingredient responsible for Saturn’s polar vortices. A 2.5-layer polar shallow-water model, introduced by O’Neill et al., is employed and described in detail. The authors first explore freely evolving behavior and then forced-dissipative behavior. It is demonstrated that local, intense vertical mass fluxes, representing baroclinic moist convective thunderstorms, can become vertically aligned and accumulate cyclonic vorticity at the pole. A scaling is found for the energy density of the model as a function of control parameters. Here it is shown that, for a fixed planetary radius and deformation radius, total energy density is the primary predictor of whether a strong polar vortex forms. Further, multiple very weak jets are formed in simulations that are not conducive to polar cyclones.

Prediction of wellbore and formation temperatures during circulation and shut-in stages under kick conditions

As the kick occurred during the drilling process is a key factor for effecting the temperature distributions of wellbore and formation, the transient heat exchange mechanisms for the radical and axial directions of wellbore and formation under kick conditions were investigated. Two operation stages, namely circulation and shut-in, were taken into consideration. Based on the first law of thermodynamics, a set of transient heat transfer models were developed to accurately predict temperature profiles of wellbore and formation. The models were solved using the fully implicit finite difference method, coupled with clustering method. The results indicated that the depth of kick and casing program could alter the heat exchange efficiency of wellbore and formation, and therefore affect the temperature distribution of wellbore and formation. Additionally, circulation and shut-in stages as well as the initial and boundary conditions for each region of wellbore and formation could also affect the heat transfer mechanism, leading to the variation of distribution distance of the initial formation temperature in the surrounding wellbore. More importantly, this model fitted actual field data better than other heat transfer models. These findings could provide theoretical insights into the temperature distribution of wellbore-formation system in drilling under kick conditions.

Validation of a Mechanistic Model for Non-Invasive Study of Ecological Energetics in an Endangered Wading Bird with Counter-Current Heat Exchange in its Legs.

Mechanistic models provide a powerful, minimally invasive tool for gaining a deeper understanding of the ecology of animals across geographic space and time. In this paper, we modified and validated the accuracy of the mechanistic model Niche Mapper for simulating heat exchanges of animals with counter-current heat exchange mechanisms in their legs and animals that wade in water. We then used Niche Mapper to explore the effects of wading and counter-current heat exchange on the energy expenditures of Whooping Cranes, a long-legged wading bird. We validated model accuracy against the energy expenditure of two captive Whooping Cranes measured using the doubly-labeled water method and time energy budgets. Energy expenditure values modeled by Niche Mapper were similar to values measured by the doubly-labeled water method and values estimated from time-energy budgets. Future studies will be able to use Niche Mapper as a non-invasive tool to explore energy-based limits to the fundamental niche of Whooping Cranes and apply this knowledge to management decisions. Basic questions about the importance of counter-current exchange and wading to animal physiological tolerances can also now be explored with the model.

Modeling the performance of a PCM cooling vest considering its side effects

Purpose – The purpose of this paper is to identify and evaluate the potential cooling contribution provided by a phase change material cooling vest as part of the total heat exchange mechanism of the body and take in to account the negative side effects of wearing the cooling garments. Design/methodology/approach – In this study, the three-part system of body-garment-environment has been simulated through the finite element method and the problem of heat exchange between these three parts has been solved with the help of computer modeling. Findings – The results of this modeling showed that a large percentage of the cooling efficiency of cooling vest was neutralized by the negative effects of the vest that are weight, lack of breathability, and the effects on the thermal conductivity of the skin. Therefore, the net efficiency of the cooling vests resulted in a lower decrease in skin temperature compared to the state that the negative side effects were not included in the model. Originality/value – Cooling power obtained with the help of cooling garments have been studied in previous studies using either human tests or manikins. But, what has been addressed less in previous studies relates to the negative effects of such equipment on the comfort of body, along with their cooling effect. So it is the first time witch the effect of side effects of such equipments are studied. Also modeling the real performance of cooling garments have not been done yet.

Exergy Analysis of Waste Heat Recovery Section in Steam-Natural Gas Reforming Process

In this work, an exergy analysis is performed for the waste heat recovery section (WHRS) of the steam-natural gas reforming (SNGR) process as a major energy intensive process. Two alternate conditions are investigated to evaluate the required thermodynamic parameters: normal operating condition and increase of C2+ components in the process feed stream. At normal operating condition, the exergy efficiency of WHRS amounts to 0.58 while some 17.2 kJ energy is destructed for each mole of H2 produced. If heavier than methane components are increased in the feed up to 8.5 mole %, despite the increase of H2 production, the exergy efficiency decreases down to 0.54 and exergy destruction is reduced to 13.8 kJ for each mole of H2 produced. It is concluded that waste heat recovery has an important role on the exergy parameters of steam reforming processes, and an efficient heat exchange mechanism should be designed for this purpose. In addition, a change of reforming feed composition has a significant effect on the ...

Vascular Development for Climate Control

Early anatomists noted the close juxtaposition of arteries and veins, which modern physiologists proposed aided temperature regulation. In this issue of Developmental Cell, Kidoya and colleagues (2015) demonstrate that the secreted protein Apelin and hematopoietic-endothelial cell interactions create this arrangement, providing genetic evidence for its function as a heat-exchange mechanism.

NUMERICAL SIMULATION OF CONVECTIVE HEAT TRANSFER AND PRESSURE DROP IN TWO TYPES OF LONGITUDINALLY AND INTERNALLY FINNED TUBES

With two kinds of methods of boundary condition treatment, three-dimensional turbulent flow and heat transfer problems in two types of internally finned tubes with a blocked core-tube have been studied numerically by the realizable k−ε model. The numerical simulation results obtained from two calculation models were compared with experimental data. It was found that the simulation results obtained from the turbulent flow model are closer to the experimental values than those obtained from the laminar flow model. Meanwhile, it has been found that the critical Reynolds number for the flow that develops in internally finned tubes from a laminar flow to a turbulent one is much less than the Reynolds number for traditional bare tubes. The calculation results also indicate that the periodical ridges inside the finned tubes change the distribution of the inner flow field and temperature profile. Unlike straight tubes, in internally finned tubes, a secondary vortex flow emerges that plays a definitely destructive role for the flow boundary layer and increases the turbulent kinetic energy of the flow field. With the field synergy principle, a contrasting analysis of the intensified heat exchange mechanism for internally finned tubes and a bare annular tube was performed quantitatively. The results show that the field synergy degree of longitudinally ridged and internally finned tubes is better than that of bare annular tubes, which enhance heat transfer.

Numerical evaluation of multipass welding temperature field in API 5L X80 steel welded joints

Many are the metallurgical changes suffered by materials when subjected to welding thermal cycle, promoting a considerable influence on the welded structures thermo mechanical properties. In project phase, one alternative for evaluating the welding cycle variable, would be the employment of computational methods through simulation. So, this paper presents an evaluation of the temperature field in a multipass welding of API 5L X80 steel used for oil and gas transportation, using the ABAQUS ® software, based on Finite Elements Method (FEM). During the simulation complex phenomena are considerable including: Variation in physical and mechanical properties of materials as a function of temperature, welding speed and the different mechanisms of heat exchange with the environment (convection and radiation) were used. These considerations allow a more robust mathematical modeling for the welding process. An analytical heat source proposed by Goldak, to model the heat input in order to characterize the multipass welding through the GTAW (Gas Tungsten Arc Welding) process on root and the SMAW (Shielded Metal Arc Welding) process for the filling passes were used. So, it was possible to evaluate the effect of each welding pass on the welded joint temperature field, through the temperature peaks and cooling rates values during the welding process.

Computation fluid dynamics analysis of the Reactor Cavity Cooling System for Very High Temperature Gas-Cooled Reactors

The design of passive heat removal systems is one of the main characteristics of the modular Very High Temperature Gas-Cooled Reactors (VHTR) vessel cavity. The Reactor Cavity Cooling System (RCCS) is a key heat removal system during normal and off normal conditions. The design and validation of the RCCS is necessary to demonstrate that VHTRs can survive the postulated accidents. The commercial Computational Fluid Dynamics (CFD) STAR-CCM+/V5.02.009 code was used for three-dimensional system modeling and analysis of the RCCS. Different RCCS geometries and configurations were investigated to analyze heat exchange in the VHTR cavity. Sensitivity analyses over the RCCS cavity height and cooling panel location with respect to the reactor pressure vessel (RPV) wall were performed. The objective of the present work was to use CFD tools for addressing the behavior of the RCCS following accident conditions.Heat removal from the RPV through the RCCS system during normal and off normal conditions is accomplished through radiation, convection and conduction heat transfer modes. The interaction of all three heat exchange mechanisms makes it very challenging to have an accurate description of the RCCS heat transfer dynamics during normal and transient conditions. A systematic analysis of the cavity layout was performed to address the influence of different geometrical parameters on the balance of heat transfer across the RCCS cavity. Mesh convergence was achieved with an intensive parametric study of the different geometrical configurations and boundary conditions selected. Based on the results of previous studies, the Realizable k–ε turbulence model with Two-Layer all y+ wall treatment was used for the analyses discussed in the present work. The numerical results demonstrated that the CFD analyses can resemble the behavior of the full RCCS system from an integral effect point of view, with the main physical phenomena accurately reproduced by the CFD model.