Poor Heat Transfer Characteristics Research Articles

Production of low molecular oil from typical polyolefin plastics through molten salts thermal treatment

The resource recovery based on pyrolysis has become the mainstream disposal method for polyolefin plastics in recent years. However, the poor heat transfer characteristics and uneven temperature distribution of plastics lead to inferior product quality. To address these problems, molten salts thermal treatment was performed to conduct targeted regulation of the process. It revealed that the excellent heat storage and thermal conductivity of molten salts provide a strengthened heat source for plastics, promoting product homogenization and high-value conversion. It reduced the energy barrier of the reaction and prompted the conversion of macromolecular components to light oil. Meanwhile, the catalysis of alkali metal salts effectively inhibited the transition cracking of major oil components. The yield of pyrolysis oil increased while the valuable products gradually accumulated, such as α-olefins (ranging from 26.48 % to 31.06 %) and styrene (ranging from 48.89 % to 64.03 %). Additionally, the kinetics of hydrogen could be used to analyze the dynamic processes of plastic pyrolysis well and the activation energy was significantly reduced with molten salts. The contracting volume model was considered to be the most suitable reaction model, providing critical insight for the thermal treatment of plastics at larger scales.

Heat Exchangers Using Multi Types of Nanofluids: A Review

A heat exchanger, which may be found in cooling systems, refineries, power plants, and other facilities, is one of the frequently utilized devices in the application heat transfer process. Water, mineral oil, and ethylene glycol—which has poor heat transfer characteristics—were the typical fluids utilized in heat exchangers. Hence, nanofluid was employed to enhance the fluid's ability to transport heat in heat exchangers. The issue of enhancing heat transmission in heat exchangers with nanofluid was researched experimentally, theoretically, and experimentally. Some of the research done in this area is as follows.

Phase change heat transfer and energy storage in a wavy-tube thermal storage unit filled with a nano-enhanced phase change material and metal foams

Latent heat thermal energy storage plays a key role in the thermal management of heat transfer systems, shifting thermal loads, and developing renewable systems. A latent heat thermal energy storage (LHTES) unit can store/release a significant amount of heat in a compact space. However, the main issue of LHTES units is their poor heat transfer characteristics. Thus, the thermal response time of most LHTES is low, and they cannot absorb/release the required energy in a timely manner. Hence, the heat transfer enhancement approaches such as using nano additives, metal foams, and extended, wavy surfaces are promising approaches to improve the heat transfer capability of LHTES. The present study aims to address the impact of using wavy tubes in a composite phase change material and metal foam LHTES unit. A phase change heat transfer model based on enthalpy-porosity was introduced and solved via the finite element approach. The influence of nanoparticle volumetric fraction (VFna), tube wave amplitude (A), tube wave number (N), and the porosity coefficient (ε) was investigated on the charging time, stored energy, and heat transfer behavior of the LHTES unit. According to the findings, employing metal foams and nanoparticles enhances heat transmission and decreases charging time. A simple tube with no wavy surface produces lower pressure drop and better charging power compared to a wavy tube.

Thermal performance improvement of energy storage materials for low operating temperature solar water heating systems

Recent studies have shown the attractive energy storage capabilities of phase change materials (PCMs); however, PCMs may not be fully effective due to their poor heat transfer characteristics. Moreover, the melting point of PCMs restricts direct integration into a solar collector for low operating solar water heating (SWH) applications. This study investigates the heat transfer enhancement of PCMs as well as aiming to lower their melting point onset to expedite the latent heat storage initiation. The selected types of PCMs are paraffin waxes with melting point temperatures of 28–72 °C. In Phase I, silicone oil is selected as a heat transfer medium to be used with the PCMs to establish natural convection enhancement, which led to maximum melting point depression of approximately 3 °C in the PCM72. In Phase II, the conductive heat transfer enhancement was studied by adding nanoparticles with varying mass concentrations. From this analysis, it was determined that PCM72 with 1% multi-walled carbon nanotubes (MWCNT) has shown an increase of approximately 3.81% in thermal conductivity. The obtained results from this study can lead to overall efficiency improvement of SWH systems by employing the PCM mixtures with low melting point, while keeping the high latent heat capacity, and high thermal conductivity.

Nanofluid NiO-H2O on heat exchanger using Taguchi optimization and numerical simulation

Heat exchangers that use conventional fluids such as water, ethylene glycol, and oil, which are often used in industry, have poor heat transfer characteristics. Therefore, in this study, we employed nanofluids as a substitute for conventional fluids in a heat exchanger to improve the heat transfer characteristics. This study aims to determine the potential of the working fluid in the heat exchanger system with the addition of NiO nanofluids (sintering and unsintering), which are distributed into the water base fluid. The results were then processed using a simulation on a heat exchanger to determine the heat transfer characteristics, namely the log mean temperature difference (ΔTLMTD) using the Taguchi and CFD methods. This study used a water flow rate (discharge) of 0.45 litre min−1, 0.9 litre min−1 and 1.35 litre min−1, which is modeled in a simulation using variations of 1 tube, 2 tubes, and 3 tubes. Numerical simulations were carried out using the CFD method from Ansys 14.5 and the Taguchi optimization from Minitab 19. The experimental results showed that the best performance for ΔTLMTD was 1.35 litre min−1 using NiO-Sintering nanofluid with 1 tube.

Performance of Supercritical CO2 Power Cycle and Its Turbomachinery with the Printed Circuit Heat Exchanger with Straight and Zigzag Channels

Since printed circuit heat exchangers (PCHE) are the largest modules of a supercritical carbon dioxide Brayton cycle, they can considerably affect the whole system’s performance and layout. Straight-channel and zigzag-channel printed circuit heat exchangers have frequently been analyzed in the standalone mode and repeatedly proposed for sCO2−BC. However, the impact of heat exchanger designs with straight and zigzag-channel configurations on the performance of the cycle and its components, i.e., the turbine and compressor, has not been studied. In this context, this study evaluates the effect of different heat exchanger designs with various values of effectiveness (ϵ), inlet Reynolds number (Re), and channel configuration (zigzag and straight channel) on the overall performance of the sCO2−BC and its components. For the design and analysis of PCHEs, an in-house PCHE design and analysis code (PCHE-DAC) was developed in the MATLAB environment. The sCO2−BC performance was evaluated utilizing an in-house cycle simulation and analysis code (CSAC) that employs the heat exchanger design code as a subroutine. The results suggest that pressure drop in PCHEs with straight-channel configuration is up to 3.0 times larger than in PCHEs with zigzag-channel configuration. It was found that a higher pressure drop in the PCHEs with straight channels can be attributed to substantially longer channel lengths required for these designs (up to 4.1 times than zigzag-channels) based on the poor heat transfer characteristics associated with these channel geometries. Thus, cycle layouts using PCHEs with a straight-channel configuration impart a much higher load (up to 1.13 times) on the recompression compressor, this in turn, results in a lower pressure ratio across the turbine. Therefore, the overall performance of the sCO2−BC using PCHEs with straight-channel configurations is found to be substantially inferior to that of layouts using PCHEs with zigzag-channel configurations. Finally, optimization results suggest that heat exchanger’s design with inlet Reynolds number and heat exchanger effectiveness ranging from 32 k to 42 k and 0.94>ϵ>0.87, respectively, are optimal for sCO2−BC and present a good bargain between cycle efficiency and its layout size.

EXPERIMENTAL INVESTIGATION ON THERMAL PERFORMANCE OF THERMOSYPHON HEAT PIPE USING DOLOMITE/DEIONIZED WATER NANOFLUID DEPENDING ON NANOPARTICLE CONCENTRATION AND SURFACTANT TYPE

The conventional fluids used for heat transfer cannot provide the desired thermal performance due to their poor heat transfer characteristics. Disadvantages of the base fluids can be overcome by forming a suspension of nanoparticles with the base fluid. In this work, the effects of using dolomite/deionized water nanofluid on the thermal performance of thermosyphon heat pipe were experimentally investigated. Nanoparticle concentration and surfactant type (sodium dodecyl benzene sulfonate (SDBS), Triton X-100) were studied as parameters. Three different cooling water mass flow rates (5, 7.5, and 10 g/s) were used in experiments with different heating powers (200, 300, and 400 W) to test heat pipe performance. The results indicated that nanoparticle concentration is an effective parameter in the performance of nanofluids, and SDBS exhibit more favorable characteristics than Triton X-100 as a surfactant. The 36.84% decrease in thermal resistance was observed compared to deionized water and enhancement of 38.75% was achieved in heat pipe efficiency using of dolomite nanofluid containing 2% of dolomite nanoparticle and 0.5% of SDBS under a heating power of 200 W and cooling water mass flow rate of 5 g/s.

Design and CFD analysis of an industrial fan for a concentrating high-temperature solar powerplant

The increasing request for the use of renewable energy sources leads to the study of different types of new alternative powerplants, one of them being the electricity generation by means of solar concentration powerplants with open volumetric receiver, using non-pressurized air as the heat transfer fluid. Due to the poor heat transfer characteristics of the fluid, a large air mass flowrate is needed in the solar tower to transfer a sufficient amount of heat to a conventional steam cycle. The aim of this study is to design a machine with 5 MW electrical power output and 1.125 compression ratio that is able to work in this severe environment. The compression needed by the forced convection circuit, requires verification of the relevance of the compressibility phenomena. The design start from the machine type selection and the sizing of the compressor, based on the standard mono-dimensional turbomachinery theory, and continue with the verification of the fluid dynamic performance of the fan, based on a commercial CFD code (ANSYS-FLUENT). A first 2D case is evaluated to ensure that the geometry is working correctly, then the full 3D geometry is simulated to quantify the real performance of the compressor. A preliminary structural analysis of the blade is also performed to verify the structural integrity of the chosen configuration.

On the dynamic modeling and control of the cold-end system in a direct air-cooling generating unit

Direct air-cooling is considered as a promising technique in power generation because of its distinct water-saving advantages, where ambient air is utilized instead of water, to cool down the exhaust steam in the direct air-cooling condenser. However, the relatively poor heat transfer characteristics and thermodynamic properties of air also brings higher backpressure in the direct air-cooling generating unit compared with conventional water-cooling generating unit, as well as the sensitivity of condenser performance to meteorological and ambient conditions. Therefore, the unit backpressure control for the direct air-cooling generating unit has been a challenging task in the recent years. To this end, a control-oriented mechanism dynamic cold-end system model is built for the direct air-cooling generating unit in this paper. The modified moving boundary method is utilized to capture the dynamic process of exhaust steam condensation in the condenser, wherein a time-variant mean void fraction is adopted to enhance model accuracy and robustness. Validation results with operating data have shown the established model can capture the dynamics of direct air-cooling cold-end system with satisfied accuracy. Based on the established model, a model predictive controller with feedforward compensation is proposed to keep unit backpressure steady at the desired set-point while suppressing the time-varying stochastic ambient temperature disturbances by regulating the rotation speed of fan array appropriately. The simulation results have verified the merits and effectiveness of the proposed strategy in achieving both satisfactory backpressure regulation and disturbance rejection performance.

Advanced materials for solid state hydrogen storage: “Thermal engineering issues”

Hydrogen has been widely recognized as the “Energy Carrier” of the future. Efficient, reliable, economical and safe storage and delivery of hydrogen form important aspects in achieving success of the “Hydrogen Economy”. Gravimetric and volumetric storage capacities become important when one considers portable and mobile applications of hydrogen. In the case of solid state hydrogen storage, the gas is reversibly embedded (by physisorption and/or chemisorption) in a solid matrix. A wide variety of materials such as intermetallics, physisorbents, complex hydrides/alanates, metal organic frameworks, etc. have been investigated as possible storage media. This paper discusses the feasibility of lithium– and sodium–aluminum hydrides with emphasis on their thermodynamic and thermo-physical properties. Drawbacks such as poor heat transfer characteristics and poor kinetics demand special attention to the thermal design of solid state storage devices.

Carbon nanohorns-based nanofluids as direct sunlight absorbers

The optimization of the poor heat transfer characteristics of fluids conventionally employed in solar devices are at present one of the main topics for system efficiency and compactness. In the present work we investigated the optical and thermal properties of nanofluids consisting in aqueous suspensions of single wall carbon nanohorns. The characteristics of these nanofluids were evaluated in view of their use as sunlight absorber fluids in a solar device. The observed nanoparticle-induced differences in optical properties appeared promising, leading to a considerably higher sunlight absorption. We found that the thermal conductivity of the nanofluids was higher than pure water. Both these effects, together with the possible chemical functionalization of carbon nanohorns, make this new kind of nanofluids very interesting for increasing the overall efficiency of the sunlight exploiting device.

Effect of Mixing During Fermentation in Yogurt Manufacturing

In traditional yogurt manufacturing, the yogurt is not agitated during fermentation. However, stirring could be beneficial, particularly for improving heat and mass transport across the fermentation tank. In this contribution, we studied the effect of low-speed agitation during fermentation on process time, acidity profile, and microbial dynamics during yogurt fermentation in 2 laboratory-scale fermenters (3 and 5L) with different heat-transfer characteristics. Lactobacillus bulgaricus and Streptococcus thermophilus were used as fermenting bacteria. Curves of pH, lactic acid concentration, lactose concentration, and bacterial population profiles during fermentation are presented for static and low-agitation conditions during fermentation. At low-inoculum conditions, agitation reduced the processing time by shortening the lag phase. However, mixing did not modify the duration or the shape of the pH profiles during the exponential phase. In fermentors with poor heat-transfer characteristics, important differences in microbial dynamics were observed between the agitated and nonagitated fermentation experiments; that is, agitation significantly increased the observable specific growth rate and the final microbial count of L. bulgaricus.

Polarisation-independence of femtosecond laser machining of fused silica

Laser direct processing of glass has been challenging due to the brittle nature and poor heat transfer characteristics of the material. With the advancement of ultrashort pulse lasers, laser direct machining of glass with smooth surface and free of microcracks has become a potential manufacturing technique in microfabrication of optical devices. In this paper, machining of microgrooves on fused silica substrates using a p-polarised femtosecond laser beam was investigated with regards to the groove profile and surface morphology. Microchannels with width and spacing of 20μm were produced with no microcracks. Repeatable sub-micron gratings within a microchannel along the cutting direction were also observed. These microchannels and gratings may be used for the manufacture of optical and microfluidic devices. The beam polarisation was found to have little effect on the machining results.

The influence of heat transfer on reduced-time plots

A theoretical approach has been used to show that, except for certain types of reaction mechanism, the ease with which it is possible to distinguish the form of the reaction mechanism by the reduced-time plot method depends particularly on the rate of transfer of heat into the sample. The original reduced-time plots [1] were calculated from model equatioons which assume that the sample is, from the outset, at a fixed temperature and remains under isothermal conditions throughout the reaction. The variations produced in the appearance of reduced-time plots when the sample is programmed to rise to a given fixed temperature through various temperature schedules have been investigated. It is shown that even relatively rapid temperature rises can produce distortion of the reduced-time plots for various reaction equations. If the reaction mechanism is known, however, fairly accurate values of the activation energy for the reaction can be determined, even when the furnace used has relatively poor heat-transfer characteristics.

Designs of metal hydride reactors

As metal hydride beds have extremely poor heat transfer characteristics, the configurations of hydride reactors will have a marked effect on the performance and cost effectiveness of these metal-hydrogen systems. Therefore, it is of great significance to develop these reactors into the most suitable configurations for optimum heat transfer performance. The results of comparative studies of one-dimensional reactors and two-dimensional reactors can be used as guides to design reactors optimally and to augment heat and mass transfer processes in metal hydride beds. The comparative study of the three elementary reactors reveals the fact that the 011 reactor has an excellent heat and mass transfer performance and reactors with similar configurations may be recommended to be used in practical metal-hydrogen systems.

Thermal Effects in a Coated Medium (with a Cavity) Due to Friction Heating by a Passing Asperity

The present paper expounds on the effect of a near-surface cavity, when the solid surface is subjected to the Coulomb frictional loading of an asperity moving at moderately high speed. The medium under consideration is represented by a solid half-space which is coaled with a thin layer of solid wear coating. The cavity is assumed to be rectangular in cross section. The temperature field and its gradient in the vicinity of the cavity are obtained during the traverse of the asperity over the surface near the cavity. The cavity defect results in a material nonuniformity mathematically modelled in term of the material coordinates. The resulting governing differential equation is time-explicit and transient. A general finite difference formulation is developed, from which numerical solutions are obtained for problem with a cavity at various positions relative to the surface-layer/substrate interface. Because of the poor heat transfer characteristics of tile cavity, the temperatures in the surface layer above i...

Thermal energy storage using saturated salt solutions

In order to reduce the volume required to store low grade thermal energy in water, various systems using phase change materials (PCMs) have been proposed. However, in order to overcome the poor heat transfer characteristics associated with the solidification of the PCM on heat transfer surfaces, large surface areas need to be provided. This is often achieved by encapsulation in either large or small containers, which has the effect of increasing the cost and reducing the effective energy density. Furthermore PCMs can only accept and release heat at one particular temperature. In an attempt to overcome these limitations thermal energy storage using saturated salt solutions has been examined. The energy density for a number of promising salts has been calculated and confirmed by experiment. Energy density increases of up to 4 times that of water are possible, depending on the salt used and the temperature swing permitted. The deposition of crystals from the solution on heat exchanger surfaces has been overcome by the use of a novel self-cleaning technique.

Heat Transfer in Processing and Use of Rubber

Abstract Heat transfer in rubber is likely to be significant from the first processing steps to the end use of the product. At the start, it is encountered in preparing, drying, and handling raw polymers; then in mixing and forming or molding compounds. In this stage, the thermoplastic nature of rubber is such that viscosity and other physical characteristics are especially temperature sensitive. Heat is the most important processing agent. The rate at which heat can get into and out of rubber, how expertly it can be transferred, affects the design of processing machinery and controls the speed of many mixing, extruding, and molding operations. In these processes, primary, turbulent heat transfer between rubber and steel and mass transfer in the rubber are complicated by frictional heat generation at the rubber-metal surfaces and large conversion of mechanical energy into internal heat. Convection heat transfer with air, steam, or fluids also usually occurs in processing rubber. Localized overheating at any stage can be disastrous for quality. Hence the basic heat problems in processing usually lie in rapidly and efficiently securing, and then maintaining, satisfactory temperature uniformity in a material with inherently poor heat transfer characteristics. Dispersing fillers in rubber requires large power inputs but the resulting temperature rise must not be so great as to cause prevulcanization or reduce unduly the efficiency of the mixing operation by too great reduction in plasticity and hence shearing stresses. On the other hand, Hahn has pointed out how advantageous such mechanical heating of rubber may be. A few minutes working on a mill may accomplish more in raising the temperature than hours of conduction heat flow. After rubber is mixed and formed, vulcanization ensues with flow of heat to raise the temperature throughout to the vulcanization range and thus activate the chemical reactions of vulcanization. At this stage, control of heat is exceedingly important for costs and quality. Any shortening of the vulcanizing cycle without detriment to quality provides opportunity to increase productivity of a large capital investiment. Finally, heat is one of the most destructive agents for finished rubber products. It presents a frontier for development of new rubbers and applications. External environmental heat imposes service limitations and in dynamic uses involving repeated, rapid deformations internal transformation of mechanical energy into heat may readily destroy thick rubber sections.