Heat Conduction Resistance Research Articles

海水塩析出物を伴う海水の流動沸騰熱伝達に関する研究

Seawater was injected into reactor cores in the accident at the Fukushima Daiichi Nuclear Power Station. This study intended to provide base data to consider reactor core cooling by seawater. Pool nucleate boiling heat transfer experiments and vertical upward forced-convective boiling heat transfer experiments were conducted by using water and artificial seawater. In the pool nucleate boiling experiments of seawater, slow surface temperature excursion was initiated at some heat flux that was considerably lower than the critical heat flux although the surface heating rate was kept almost constant. The initiation of the surface temperature excursion coincided with the initiation of the deposition of calcium sulfate on the heat transfer surface. The temperature excursion was caused by the heat conduction resistance increase with an increase in deposition layer thickness. It was suggested that the deposition of calcium sulfate on the heat transfer surface starts when the seawater concentration at the vicinity of the heat transfer surface becomes lower than 11 wt%. In the forced-convective boiling heat transfer, even if the seawater concentration at the inlet was low, the sea salt concentration was enriched because of evaporation as flow proceeded, which resulted in the initiation of the surface temperature excursion.

Corrosion Protection Mechanism of Novel Copper Alloy Resistant to Formicary Corrosion

Copper tubes are widely used in heat exchangers for air conditioning apparatus owing to their superior heat conductivity, formability, and corrosion resistance. However, in spite of their remarkable corrosion performance, copper tubes could undergo a specific type of indoor corrosion called “formicary corrosion”. This is still the most important concern of using copper tubes because increasing numbers of early failure are considered to be due to this attack. Formicary corrosion, also called “ant nest corrosion”, is a form of localized copper corrosion that progresses insidiously and rapidly. The corrosion morphology is literally ant-nest-like shape, resulted from randomly branched and interconnected small tunnels that propagate independently of grain boundary and crystallographic orientation. Early detection is difficult because the opening of the corrosion pit is too small to be seen with unaided eye. Formicary corrosion is caused by organic acids, typically formic acid and acetic acid, which are often released from construction materials. Thus, there is a growing interest in development of a new copper alloy resistant to formicary corrosion. It is widely recognized that phosphorus deoxidized copper (PDC) containing a slight amount of phosphorus (0.015 to 0.040%) is more sensitive to formicary corrosion than oxygen free copper (ODC). However, our group have extensively studied the effect of phosphorus content on the corrosion sensitivity and discovered that more than 0.2mass% of phosphorus addition dramatically enhances the corrosion resistance without spoiling mechanical properties, such as drawability and form rollability. Copper alloys that comprise 0.22 to 0.29 mass% of phosphorus were formed to inner grooved tubes with 0.25 mm wall thickness. PDC and OFC tubes were also prepared for control. To assess the formicary corrosion resistance, specimens were suspended in 2L plastic bottles with 100 ml of 0.01 or 0.1vol% formic acid so that the specimens were exposed to formic acid vapor. These bottles were thermally cycled between 40 ºC for 22 hours and room temperature for 2 hours except weekend. After corrosion tests, corrosion products were removed using 5% sulphuric acid. Dye penetrant inspection was conducted to detect the tiny corrosion pits. Strongly dyed points were chosen and cross-sectioned to examine the corrosion morphology and pit depth. It is noteworthy that the strength of the indication is proportional to the pit depth. If the pit penetrates the wall thickness, the indication is distinctively strong and easy to distinguish. Cross-sectional observation revealed that under 0.1vol% formic acid environment both PDC and OFC suffered typical and severe formicary corrosion, penetrating the tube wall less than 20 days. However, even after 80 days of exposure, corrosion depths of the new alloy tubes were hampered below 150 micrometer. Furthermore, their corrosion morphologies were wide and shallow pitting-like attack, which was clearly different from formicary corrosion. The corrosion inhibition effect of phosphorus is attributed to the formation of phosphoric acid released from the alloy, as demonstrated by ion chromatography and EPMA.

Investigation of micro- and nanostructured coatings for heat exchanger surfaces in an ice store

During loading ice stores an ice layer is forming on the heat exchanger surface when the surface temperature is coming below the nucleation temperature. The growing ice layer increases the heat conduction resistance and the loading performance decreases. With the application of micro- and nanostructured coatings the nucleation temperature of water drops on surfaces in an atmosphere of air can be decreased. If the coatings show similar characteristics under water, they can be used for heat exchanger surfaces in ice stores. Ideally the water in the ice store can be supercooled while ice growth is initiated at nucleation spots apart of the heat exchanger surface. Then a higher loading performance and storage capacity can be realized. First experiments have shown that the coatings can decrease the nucleation temperature.

Numerical Simulation of Diesel Particulate Filter Regeneration Considering Ash Deposit

Non-combustible ash can be deposited on channel walls through the full length of a diesel particulate filter (DPF). This type of ash can affect the exhaust condition and heat transfer process during the periodical soot regeneration of DPF. A model of soot regeneration is established in this paper to describe the effects of ash deposits on exhaust condition and heat transfer. Mass, momentum, and energy balances are considered for the multiphase system of gas, soot, and wall. The good agreement with experimental data confirms that the model is able to describe the processes. The axial patterns of wall temperature and soot regeneration rate during DPF regeneration are investigated by numerical simulation. Results indicate that the deposited ash layer reduces the exhaust flow rate and increases the heat conduction resistance during DPF regeneration, thus leading to high wall temperature and soot oxidation rate. When 5 g/L of ash is deposited, the complete oxidation of soot can be achieved 90 s faster with a 60 ∘C increase in wall temperature. The gain on soot oxidation rate increases with increasing amounts of deposited ash and reaches a plateau when the deposited ash approaches 15 g/L. Owing to the sintering and melting of ash when the temperature reaches 900 ∘C and the consequent uncontrolled regeneration, the soot carrying capacity should be identified based on the amount of deposited ash within DPF.

Verification of a model of thermal storage incorporated with an extended lumped capacitance method for various solid–fluid structural combinations

With an extended lumped capacitance method applied to account for the internal heat conduction resistance in a solid (for Biot number larger than 0.1), a general model of thermal energy storage with various solid–fluid structural combinations is presented and verified using numerical results. The thermal energy storage system has a heat transfer fluid (HTF) flowing through a packed bed of solid materials structured in different configurations, such as in the form of solid pebbles, parallel plates, solid rod-bundles, or solids with fluid tubes imbedded through them. The model of energy conservation in the liquid and solid is transient, one-dimensional in nature, due to the introduction of a modified lumped capacitance method that counts for the effect of three-dimensional heat conduction in the solid structures. The computational workload using this modified model is significantly less compared to that of a comprehensive CFD analysis. Numerical results obtained from a CFD analysis of the thermal energy storage in the solid and liquid are used to verify the model. The CFD simulated results of temperatures of HTF are compared with the 1D model results, and they show excellent agreement. In conclusion, the 1D model is recommended as a convenient and accurate tool for general analysis and sizing of thermal energy storage containers that have various solid–fluid structural combinations.

Review: Effect of Alloying Element on Al-Si Alloys

Al-Si alloys are the most common aluminium cast products owing to their high resistance to hot cracking and excellence fluidity during the molten state. They play important roles in aerospace, automobile and structural industries where high strength to weigh ratio, superior heat conductivity and good corrosion resistance applications are necessary. Alloying elements such as copper, magnesium, zinc and nickel are added into the Al-Si alloys for further strength enhancement. However, impurities such as iron are often present in Al-Si alloys, forming brittle intermetallic phases and hence reducing the mechanical strength of the alloys. In this paper, previous studies on the effects of alloying elements on physical and mechanical characteristics of Al-Si alloys will be accounted. Moreover, a comprehensive review on the Al-Si alloy casting methods will also be included.

Thermal Simulation of Plate-Fin Heat Exchangers

Plate-fin heat exchangers are playing an important role in the power and process industry in the recent past. Hence, it has become necessary to model their temperature response accurately. A new mathematical model of plate-fin heat exchanger is proposed, considering the heat conduction resistance along the fins and the separating plates. The effects of fin geometrical parameters and spacer thickness on the performances of the heat exchanger have been numerically calculated. It is found that the heat conduction resistance of fins has significant influence on the outlet fluid temperature variation. The analysis presented here suggests a better method of heat-transfer data analysis for plate heat exchangers.

An Experimental Study on Heat Conduction and Thermal Contact Resistance for the AlN Flake

The electrical technology has been a fast development over the past decades. Moreover, the tendency of microelements and dense division multiplex is significantly for the electrical industries. Therefore, the high thermal conductible and electrical insulating device will be popular and important. It is well known that AlN still maintains stablility in the high temperature. This is quite attractive for the research and development department. Moreover, the thermal conduct coefficient of AlN is several times larger than the others. Therefore, it has been thought to play an important role for the radiator of heat source in the future. Therefore, this paper is focused on the studies of heat conduction and thermal contact resistance between the AlN flake and the copper specimens. The heating temperatures and the contact pressures were selected as the experimental parameters. According to the experimental results, the materials are soft and the real contact areas between the interfaces significantly increase under higher temperatures. As a result, the thermal contact resistance significantly decreases and the heat transfer rate increases with increasing the heating temperature or the contact pressures.

A Study of Energy Saving in Building through Thermal Insulation with Plywood Inserted Honeycomb Sandwich Panels

The objective of this research is to examine the possibility of replacing the plywood-reinforced honeycomb sandwich panel for the conventional honeycomb sandwich panel as building materials. Three types of plywood were selected for study, i.e., chinese plywood, rubber plywood, and teak plywood, and the finite element simulation was performed to investigate the heat transfer. The result of the simulation showed that inserting plywood into the sandwich material could reduce the heat transfer content, especially the sandwich material inserted with chinese plywood which yielded the most efficient insulation, followed respectively by with teak plywood and rubber plywood. The result conformed to the heat conduction and heat resistance of the ASTM C 518 standards; therefore, the chinese plywood sandwich panels were used to construct the walls of a building so as to study the energy saving efficiency inside the building. The experimental result showed that the energy consumption content in the room with regular walls was 1.07 kw., whereas the room with the chinese plywood sandwich panel walls consumed energy merely 0.90 kw. The employment of chinese plywood sandwich panel walls thereby reduced the energy consumption by 0.17 kw. In other words, the room walled off with the sandwich panel walls could save more energy than that with regular walls.

Thin Flooring Panel Development for Energy Efficiency of an Ondol Heating System

According to a Seoul Housing Corporation report, due to low heat conductivity of wood insulation, management difficulties have occurred in wood flooring panels, one of the main panels used in the Korean Ondol heating system used in Korean housing. Although many researchers have studied flooring panels to date, they have not sufficiently addressed heat conductivity and bending resistance improvements. This research aims to improve heat conductivity of conventional panels, developing thin flooring panels for better heating energy conduction. The panel that this research proposes would provide economic benefits by decreasing the panel thickness. In addition, improvement in bending resistance was found to prevent wood shrinkage. A comparison with conventional panels shows a 30% increase in heat conductivity, and 7.2% reduction of energy consumption. The panel that this research proposes will be applicable effectively to the Korean Ondol system.

3D Integrated Water Cooling of a Composite Multilayer Stack of Chips

New generation supercomputers with three dimensional stacked chip architectures pose a major challenge with respect to the removal of dissipated heat, which can reach currently as high as 250 W/cm2 in multilayer chip stacks of less than 0.3 cm3 volume. Interlayer integrated water cooling is a very promising approach for such high heat flux removal due to much larger thermal capacity and conductivity of water compared with air, the traditional cooling fluid. In the current work, a multiscale conjugate heat transfer model is developed for integrated water cooling of chip layers and validated with experimental measurements on an especially designed thermal test vehicle that simulates a four tier chip stack with a footprint of 1 cm2. The cooling heat transfer structure, which consists of microchannels with cylindrical pin-fins, is conceived in such a way that it can be directly integrated with the device layout in multilayer chips. Every composite layer is cooled by water flow in microchannels (height of 100 μm), which are arranged in two port water inlet-outlet configuration. The total power removed in the stack is 390 W at a temperature gradient budget of 60 K from liquid inlet to maximal junction temperature, corresponding to about 1.3 kW/cm3 volumetric heat flow. The computational cost and complexity of detailed computational fluid dynamics (CFD) modeling of heat transfer in stacked chips with integrated cooling can be prohibitive. Therefore, the heat transfer structure is modeled using a porous medium approach, where the model parameters of heat transfer and hydrodynamic resistance are derived from averaging the results of the detailed 3D-CFD simulations of a single streamwise row of fins. The modeling results indicate that an isotropic porous medium model does not accurately predict the measured temperature fields. The variation of material properties due to temperature gradients is found to be large; therefore, variable properties are used in the model. It is also shown that the modeling of the heat transfer in the cooling sublayers requires the implementation of a porous medium approach with a local thermal nonequilibrium, as well as orthotropic heat conduction and hydrodynamic resistance. The improved model reproduces the temperatures measured in the stack within 10%. The model is used to predict the behavior of multilayer stacks mimicking the change of heat fluxes resulting from variations in the computational load of the chips during their operation.

Operational properties of a heat-insulation composite with expanded polystyrene crumbs

The operational properties (strength, heat conductivity, hygroscopic sorption, vapor permeability, and frost resistance) of a heat-insulation composite of density 150–350 kg/m3 are investigated. The matrix consists of an aerated concrete substance and a disperse filler of expanded polystyrene crumbs obtained by crushing the household polystyrene package waste. The macro-and microstructure of the composite are considered. It is found that the operational properties of the material directly depend on its density, determined by the amount of the cementing substance in the matrix.

Kinetics of reduction and oxidation reactions for application in catalytic gas–particle-filters

Catalytic filters offer the possibility to combine the functions of a catalytic reactor and a gas–particle filter within one process step. In this way, plant complexity may be reduced, which is of economical interest for small- to medium-size biomass combustion plants. SiC materials seem attractive for catalytic activation as they exhibit high heat conductivity and good thermal shock resistance properties. Furthermore, SiC filter candles are coated with a dense mullite membrane (SiO 2–Al 2O 3 coating), which protects the inner porous, catalytic filter support material against any catalyst poisons carried by the particles. Within this research, filter candles based on SiC, manufactured and impregnated with V 2O 5–TiO 2–WO 3 catalysts by Pall–Schumacher, are investigated with respect to catalytic reactions in gas cleaning. For a better kinetic analysis also accordingly catalytic impregnated SiC grains were investigated in a fixed bed reactor.

Single-step laser deposition of functionally graded coating by dual ‘wire–powder’ or ‘powder–powder’ feeding—A comparative study

The creation of iron–copper (Fe–Cu) alloys has practical application in improving the surface heat conduction and corrosion resistance of, for example, conformal cooling channels in steel moulds, but is difficult to achieve because the elements have got low inter-solubility and are prone to solidification cracking. Previous work by these authors has reported a method to produce a graded iron–nickel–copper coating in a single-step by direct diode laser deposition (DLD) of nickel wire and copper powder as a combined feedstock. This work investigates whether dual powder feeds can be used in that process to afford greater geometric flexibility and compares attributes of the ‘nickel wire and copper powder’ and ‘nickel powder and copper powder’ processes for deposition on a H13 tool steel substrate. In wire–powder deposition, a higher temperature developed in the melt pool causing a clad with a smooth gradient structure. The nickel powder in powder–powder deposition did not impart much heat into the melt pool so the melt pool solidified with sharp composition boundaries due to single metal melting in some parts. In wire–powder experiments, a graded structure was obtained by varying the flow rates of wire and powder. However, a graded structure was not realised in powder–powder experiments by varying either the feed or the directions. Reasons for the differences and flow patterns in the melt pools and their effect on final part properties of parts produced are discussed.

Evaporation Heat Transfer in Sintered Porous Media

A two-dimensional model is presented to predict the overall heat transfer capability for a sintered wick structure. The model considers the absence of bulk fluid at the top surface of the wick, heat conduction resistance through the wick, capillary limitation, and the onset of nucleate boiling. The numerical results show that thin film evaporation occurring only at the top surface of a wick plays an important role in the enhancement of evaporating heat transfer and depends on the thin film evaporation, the particle size, the porosity, and the wick structure thickness. By decreasing the average particle radius, the evaporation heat transfer coefficient can be enhanced. Additionally, there exists an optimum characteristic thickness for maximum heat removal. The maximum superheat allowable for thin film evaporation at the top surface of a wick is presented to be a function of the particle radius, wick porosity, wick structure thickness, and effective thermal conductivity. In order to verify the theoretical analysis, an experimental system was established, and a comparison with the theoretical prediction conducted. Results of the investigation will assist in optimizing the heat transfer performance of sintered porous media in heat pipes and better understanding of thin film evaporation.

D232 マイクロ・ベアチューブ熱交換器の電子機器冷却への適用性評価

In the present study, a micro bare-tube heat exchanger without conventional fins is proposed and evaluated for electronic equipment cooling application. A micro bare-tube heat exchanger composed of 0.5mm outer diameter copper tubes is manufactured and tested experimentally. The optimal dimensionless transverse and longitudinal tube pitches were PT = 2.28 and PL = 1.31, respectively. It is shown that the resultant micro bare-tube heat exchanger can drastically reduce the core volume compared to the conventional plate fin and tube heat exchanger. INTRODUCTION Great efforts have been made for heat transfer augmentation, and a number of design concepts for compact heat exchangers have been proposed (e.g., see Kays and London [1]). Most of the gas-liquid heat exchangers adopt fins in order to compensate a lower heat transfer rate on the gas side. Paitoonsurikarn et al. [2] proposed a micro bare-tube heat exchanger, which was composed of a bundle of small diameter tubes without conventional fins. It was shown that the micro bare-tube heat exchanger had a possibility of improving heat exchanging performance and compactness with its high over-all heat transfer rate and large heat transfer area density. Conventional liquid-air heat exchangers such that used for air conditioners and electronic equipment coolers generally require large heat transfer area and air flow rate because of the small thermal conductivity of air. This often increases the volume, noise and also initial and running costs of the total system. On the other hand, the micro bare-tube heat exchanger has a potential for improving air-side performance drastically, because both the heat conduction resistance of fins and the inner-tube heat resistance can be ignored compared to those of conventional heat exchangers. Moreover, it is possible to achieve a large heat transfer area density. This feature is really desirable for designing compact cooling systems. Paitoonsurikarn et al. [2] utilized the heat transfer and pressure drop correlations proposed by Zukauskas [3]. However, those correlations are not verified for predicting a wide range of tube arrangements and also at low Reynolds numbers, e.g., Re < 500, which is a characteristic Re range for compact heat exchangers. In the present study, optimization method based on simulated annealing (SA) with a trained neural network representing the heat transfer and pressure drop characteristics of a prescribed tube bank is introduced. A commercial CFD code, FLUENT5, was employed to train the neural network of this flow and thermal fields around various in-line tube bundle arrangements at low Reynolds numbers. Then an optimal micro bare-tube heat exchanger composed of 0.5 (mm) outer diameter copper tubes was manufactured and tested experimentally. The optimal dimensionless transverse and longitudinal tube pitches were PT = 2.28 and PL = 1.31, respectively. The experimental data were then directly utilized for the assessment of CPU cooling system. It is shown that the resultant micro bare-tube heat exchanger can drastically reduce the core volume compared to the conventional heat exchangers. NUMERICAL SIMULATION OF FLOW AROUND IN-LINE TUBE BUNDLES A commercial CFD code, FLUENT5, is employed to calculate the flow and thermal field around in-line tube bundles with three rows in the transverse direction and 10 columns in the longitudinal direction as shown in Fig. 1. The tube surface is assumed to be isothermal. Periodic boundary condition is employed in the transverse direction, and uniform velocity and free outflow conditions are given at inlet and outlet boundaries, respectively. The temperature dependence of the physical properties of working fluids is neglected. The Reynolds number based on the tube diameter and the maximum velocity at the minimum cross section, Remax, is varied in the range of Remax=10 to 300. The dimensionless transverse and longitudinal tube pitches, PT and PL, are varied from 1.25 to 4.5. In total, 46 cases are calculated. It is known that three dimensionality of the flow is observed in the case of a single cylinder around Remax =100. In the present study, however, two-dimensional unsteady calculation is performed because it is considered that the effect of three dimensionality of the flow on the mean heat transfer and pressure drop is negligibly small. Finally, the Nusselt number Nui and the pressure coefficient Cp at i-th column are obtained by time 10d 40d d PL・d PT・d Outlet Inlet Periodic Boundary Fig. 1. Computational Domain.

Heat transfer in high density electronics packaging

In order to get an insight into the thermal characteristic and to evaluate the thermal reliability of the “System in Packaging” (SIP), a new solution of electronics packaging, a heat transfer model of SIP was developed to predict the heat dissipation capacity and to investigate the effect of different factors on the temperature distribution in the electronics. The affecting parameters under consideration include the thermophysical properties of the substrates, the coefficient of convection heat transfer, the thickness of the chip, and the density of power dissipation. ALGOR, a kind of finite element analysis software, was used to do the model simulation. Based on the simulation and analysis of the heat conduction and convection resistance, criteria for the thermal design were established and possible measurement for enhancing power dissipation was provided. The results show that the heat transfer model provides a new and effective way to the thermal design and thermal analysis of SIP and to the mechanical analysis for the further investigation of SIP.

The single-blow transient testing technique for plate–fin heat exchangers

A new model of the single-blow problem is proposed, considering the lateral heat conduction resistance along the fins, the axial heat conduction along the separating plates and the axial thermal dispersion in the fluid. For plate–fin heat exchangers made up of stainless steel, the effect of the lateral heat conduction resistance along the fins can usually not be neglected. This effect is taken into account by solving the temperature dynamics in the fluid, separating plates and fins simultaneously. The axial dispersion model is used to take flow maldistribution in plate–fin heat exchangers into account. The effect of the axial heat conduction in separating plates is also considered. The governing equation system is solved by means of Laplace transform and numerical inverse transform algorithms. The investigation confirms: for plate–fin heat exchangers of aluminium the effect of the lateral heat conduction resistance of fins can usually be neglected because of their high fin efficiency. However, the assumption of uniform porous medium is not valid if the plate–fin heat exchangers are made up of stainless steel. In such a case the heat conduction resistance of fins has significant influence on the outlet fluid temperature variation.

A Nonlinear Solution of Inverse Heat Conduction Problem for Obtaining the Inner Heat Transfer Coefficient

When steam-water two-phase mixtures flow in inclined or horizontal tubes that are heated by alternating current, the circular angle-dependent temperatures on the outer radius imply that the inner heat transfer coefficients also vary with circular angle. The inner heat transfer coefficients are difficult to measure directly, but may be determined with the aid of inverse heat conduction theory. The direct model calculates the temperature field inside a half-pipe. This is subjected to a given heat transfer coefficient angular profile on its inner radius. The inverse heat conduction model calculates the temperature field under the conditions of the measured discrete temperatures and the heat-insulated boundary on the outer radius. Variation of the cylinder heat conductivity and specific resistance versus temperatures are considered in both models. The prediction accuracy is analyzed with a numerical test. The inverse heat conduction problem solution is verified as a useful tool for obtaining the inner heat transfer coefficient.

Uni-directional freezing of waste activated sludges: Vertical freezing versus radial freezing

Radial freezing and vertical freezing of waste activated sludge were conducted in this work. In radial freezing, where the sludge sample is placed in a cylindrical chamber cooled at its outer wall surface, the ice front grows and migrates inwards from the rim region along a radial direction. The corresponding freezing speed decreases gradually due to the increase in heat conduction resistance through the ice layer. In vertical freezing, where the sludge was placed in sample tubes immersed vertically into a freezing pool, the ice front grows upwards at a constant freezing speed. Comparisons between floc characteristics and the changes in chemical compositions resulting from both tests make clear the role of freezing speed. That is, if only the developing ice front can reject foreign particles and thereby cause the gross migration of sludge flocs, the effects on sludge performance would be similar. The most influential factor that determines the freeze/thaw-conditioned sludge performance is discussed.