Hasil untuk "Heat"

Lianbin Zhang, Bo Tang, Jinbo Wu et al.

R. Shah, D. P. Sekulic

Preface. Nomenclature. 1 Classification of Heat Exchangers. 1.1 Introduction. 1.2 Classification According to Transfer Processes. 1.3 Classification According to Number of Fluids. 1.4 Classification According to Surface Compactness. 1.5 Classification According to Construction Features. 1.6 Classification According to Flow Arrangements. 1.7 Classification According to Heat Transfer Mechanisms. Summary. References. Review Questions. 2 Overview of Heat Exchanger Design Methodology. 2.1 Heat Exchanger Design Methodology. 2.2 Interactions Among Design Considerations. Summary. References. Review Questions. Problems. 3 Basic Thermal Design Theory for Recuperators. 3.1 Formal Analogy between Thermal and Electrical Entities. 3.2 Heat Exchanger Variables and Thermal Circuit. 3.3 The ?(Epsilon)-NTU Method. 3.4 Effectiveness - Number of Transfer Unit Relationships. 3.5 The P-NTU Method. 3.6 P-N TU R elat ionships. 3.7 The Mean Temperature Difference Method. 3.8 F Factors for Various Flow Arrangements. 3.9 Comparison of the ?(Epsilon)-NTU, P-NTU, and MTD Methods. 3.10 The ?(Psi)-P and P1-P2 Methods. 3.11 Solution Methods for Determining Exchanger Effectiveness. 3.12 Heat Exchanger Design Problems. Summary. References. Review Questions. Problems. 4 Additional Considerations for Thermal Design of Recuperators. 4.1 Longitudinal Wall Heat Conduction Effects. 4.2 Nonuniform Overall Heat Transfer Coefficients. 4.3 Additional Considerations for Extended Surface Exchangers. 4.4 Additional Considerations for Shell-and-Tube Exchangers. Summary. References. Review Questions. Problems. 5 Thermal Design Theory for Regenerators. 5.1 Heat Transfer Analysis. 5.2 The ?(Epsilon)-NTUo Method. 5.3 The ?(Lambda)-?(Pi) Method. 5.4 Influence of Longitudinal Wall Heat Conduction. 5.5 Influence of Transverse Wall Heat Conduction. 5.6 Influence of Pressure and Carryover Leakages. 5.7 Influence of Matrix Material, Size, and Arrangement. Summary. References. Review Questions. Problems. 6 Heat Exchanger Pressure Drop Analysis. 6.1 Introduction. 6.2 Extended Surface Heat Exchanger Pressure Drop. 6.3 Regenerator Pressure Drop. 6.4 Tubular Heat Exchanger Pressure Drop. 6.5 Plate Heat Exchanger Pressure Drop. 6.6 Pressure Drop Associated with Fluid Distribution Elements. 6.7 Pressure Drop Presentation. 6.8 Pressure Drop Dependence on Geometry and Fluid Properties. Summary. References. Review Questions. Problems. 7 Surface Basic Heat Transfer and Flow Friction Characteristics. 7.1 Basic Concepts. 7.2 Dimensionless Groups. 7.3 Experimental Techniques for Determining Surface Characteristics. 7.4 Analytical and Semiempirical Heat Transfer and Friction Factor Correlations for Simple Geometries. 7.5 Experimental Heat Transfer and Friction Factor Correlations for Complex Geometries. 7.6 Influence of Temperature-Dependent Fluid Properties. 7.7 Influence of Superimposed Free Convection. 7.8 Influence of Superimposed Radiation. Summary. References. Review Questions. Problems. 8 Heat Exchanger Surface Geometrical Characteristics. 8.1 Tubular Heat Exchangers. 8.2 Tube-Fin Heat Exchangers. 8.3 Plate-Fin Heat Exchangers. 8.4 Regenerators with Continuous Cylindrical Passages. 8.5 Shell-and-Tube Exchangers with Segmental Baffles. 8.6 Gasketed Plate Heat Exchangers. Summary. References. Review Questions. 9 Heat Exchanger Design Procedures. 9.1 Fluid Mean Temperatures. 9.2 Plate-Fin Heat Exchangers. 9.3 Tube-Fin Heat Exchangers. 9.3.4 Core Mass Velocity Equation. 9.4 Plate Heat Exchangers. 9.5 Shell-and-Tube Heat Exchangers. 9.6 Heat Exchanger Optimization. Summary. References. Review Questions. Problems. 10 Selection of Heat Exchangers and Their Components. 10.1 Selection Criteria Based on Operating Parameters. 10.2 General Selection Guidelines for Major Exchanger Types. 10.3 Some Quantitative Considerations. Summary. References. Review Questions. Problems. 11 Thermodynamic Modeling and Analysis. 11.1 Introduction. 11.2 Modeling a Heat Exchanger Based on the First Law of Thermodynamics. 11.3 Irreversibilities in Heat Exchangers. 11.4 Thermodynamic Irreversibility and Temperature Cross Phenomena. 11.5 A Heuristic Approach to an Assessment of Heat Exchanger Effectiveness. 11.6 Energy, Exergy, and Cost Balances in the Analysis and Optimization of Heat Exchangers. 11.7 Performance Evaluation Criteria Based on the Second Law of Thermodynamics. Summary. References. Review Questions. Problems. 12 Flow Maldistribution and Header Design. 12.1 Geometry-Induced Flow Maldistribution. 12.2 Operating Condition-Induced Flow Maldistribution. 12.3 Mitigation of Flow Maldistribution. 12.4 Header and Manifold Design. Summary. References. Review Questions. Problems. 13 Fouling and Corrosion. 13.1 Fouling and its Effect on Exchanger Heat Transfer and Pressure Drop. 13.2 Phenomenological Considerations of Fouling. 13.3 Fouling Resistance Design Approach. 13.4 Prevention and Mitigation of Fouling. 13.5 Corrosion in Heat Exchangers. Summary. References. Review Questions. Problems. Appendix A: Thermophysical Properties. Appendix B: ?(Epsilon)-NTU Relationships for Liquid-Coupled Exchangers. Appendix C: Two-Phase Heat Transfer and Pressure Drop Correlations. C.1 Two-Phase Pressure Drop Correlations. C.2 Heat Transfer Correlations for Condensation. C.3 Heat Transfer Correlations for Boiling. Appendix D: U and CUA Values for Various Heat Exchangers. General References on or Related to Heat Exchangers. Index.

D. Wen, Yulong Ding

L. Rizhsky, Hongjian Liang, J. Shuman et al.

Z. Su

Abstract. A Surface Energy Balance System (SEBS) is proposed for the estimation of atmospheric turbulent fluxes and evaporative fraction using satellite earth observation data, in combination with meteorological information at proper scales. SEBS consists of: a set of tools for the determination of the land surface physical parameters, such as albedo, emissivity, temperature, vegetation coverage etc., from spectral reflectance and radiance measurements; a model for the determination of the roughness length for heat transfer; and a new formulation for the determination of the evaporative fraction on the basis of energy balance at limiting cases. Four experimental data sets are used to assess the reliabilities of SEBS. Based on these case studies, SEBS has proven to be capable to estimate turbulent heat fluxes and evaporative fraction at various scales with acceptable accuracy. The uncertainties in the estimated heat fluxes are comparable to in-situ measurement uncertainties. Keywords: Surface energy balance, turbulent heat flux, evaporation, remote sensing

W. Kays, A. London, E. Eckert

J. Rouse, P. Cohen, S. Trigon et al.

L. Miró, J. Gasia, L. Cabeza

I. B. Slimen, T. Najar, A. Ghram et al.

Sarah Rothstein

A. Kasaeian, Reza Daneshazarian, O. Mahian et al.

O. Mazdiyasni, A. Aghakouchak, Steven J. Davis et al.

An increase of 0.5°C in summer mean temperatures increases the probability of mass heat-related mortality in India by 146%. Rising global temperatures are causing increases in the frequency and severity of extreme climatic events, such as floods, droughts, and heat waves. We analyze changes in summer temperatures, the frequency, severity, and duration of heat waves, and heat-related mortality in India between 1960 and 2009 using data from the India Meteorological Department. Mean temperatures across India have risen by more than 0.5°C over this period, with statistically significant increases in heat waves. Using a novel probabilistic model, we further show that the increase in summer mean temperatures in India over this period corresponds to a 146% increase in the probability of heat-related mortality events of more than 100 people. In turn, our results suggest that future climate warming will lead to substantial increases in heat-related mortality, particularly in developing low-latitude countries, such as India, where heat waves will become more frequent and populations are especially vulnerable to these extreme temperatures. Our findings indicate that even moderate increases in mean temperatures may cause great increases in heat-related mortality and support the efforts of governments and international organizations to build up the resilience of these vulnerable regions to more severe heat waves.

Pengli Lei, Baojian Ji, Jing Hou et al.

Reaction-bonded silicon carbide (RB-SiC) is the preferred material for space optical systems because of its low density and high specific stiffness. However, its hardness and multi-component properties lead to low efficiency and pit defects during the polishing process, making the fabrication of RB-SiC a significant challenge. This study proposes a high-efficiency and low-defect fabrication method for RB-SiC using center-inlet computer-controlled polishing (CCP). We first investigated the polishing efficiency and surface quality achieved with center-inlet and non-center-inlet liquids. The results show that the defect density under non-center-inlet conditions was positively correlated with process parameters, while fewer defects and higher efficiency could be achieved under center-inlet conditions. Additionally, the efficient removal and defect suppression mechanisms under the center-inlet condition were revealed based on machining force, heat, and defect characterization. Under center-inlet conditions, the friction coefficient is larger and stable, resulting in high removal efficiency. The macro–micro coupled analysis results show that pit defects are generated through the combined action of force and heat, which leads to the thermo-mechanical degradation and shedding of SiC particles due to the temperature increase in the machining zone. The results demonstrate that center-inlet CCP not only ensures sufficient abrasion at the polishing interface to achieve high removal efficiency but also significantly suppresses the processing heat, thereby resulting in a low-defect surface.

Qiguang Liu, Yanyun Li, Zhenghao Wu et al.

ABSTRACT In the era of artificial intelligence (AI)‐driven high‐performance computing, phase change materials (PCMs) are critical for high‐flux thermal management. PCMs are evolving toward high enthalpy, low interfacial thermal resistance (ITR), and high reliability. Herein, we design double‐brush phase‐change polymer (PVBS‐TMCn) crosslinked by B─O─B and Si─O─B dynamic bonds, characterized by the ultra‐fast relaxation time of 0.8 s under 80°C and closed‐loop cycling. This architecture enhances the content of phase‐change units for elevated theoretical enthalpy, while inherent multiple dynamic bonds and ultra‐low entanglement minimize enthalpy loss, resulting in a record enthalpy of 240.7 J·g−1. Furthermore, a composite of flexibility PVBS‐TMC14/24 and graphene foam films (PVBS‐TMC/GF) is fabricated as thermal interface materials using a stacking‐cutting strategy, which self‐adaptively modulates low‐ITR in response to temperature, owing to phase transition properties, ultra‐low modulus, and adaptive filling capability of dynamic polymer matrix. PVBS‐TMC/GF significantly generates better thermal management efficiency compared to commercial products. The topology design of double‐brush polymer dynamic networks and interfacial contact mechanisms provide fundamental insights for developing phase‐change adaptive materials and advancing thermal management.

Jian Li, He-Nan Bao, Ke-Yu Li et al.

The rose, often referred to as the queen of flowers, is one of the four primary cut flowers; however, its growth is susceptible to high-temperature stress. Furthermore, with the advent of global warming, extreme high temperatures are becoming increasingly frequent, presenting significant challenges to the normal growth of roses. High temperatures have emerged as a limiting factor in rose cultivation. In this study, transcriptomics combined with lipid determination was employed to reveal that high temperatures resulted in a significant enrichment of genes within the α-linolenic acid metabolic pathway. Subsequent lipid determination analyses indicated that, following high-temperature treatment of the rose variety 'Hi-Ohgi', the content of α-linolenic acid increased, while there was a notable decrease in the content of the chloroplast lipid components monogalactosyldiacylglycerol (MGDG) and digalactosyldiacylglycerol (DGDG), alongside an increase in jasmonic acid content. In comparison to the heat-susceptible variety 'Scarlet Bonica', the heat-resistant rose variety 'Hi-Ohgi' exhibited lower levels of MGDG and DGDG, but higher levels of α-linolenic acid and jasmonic acid, with this trend becoming more pronounced under high-temperature conditions. Additionally, pretreatment of suspension cells from 'Scarlet Bonica' with α-linolenic acid or jasmonic acid significantly inhibited the burst of reactive oxygen species induced by high temperatures. This suggests that roses enhance the production of α-linolenic acid and jasmonic acid through lipid remodeling as a response to high-temperature stress. This research provides a theoretical foundation and precise targets for the cultivation of heat-resistant rose varieties.

A. Faghri, Yuwen Zhang

Devendra Kumar, Jagdev Singh, Kumud Tanwar et al.

Abstract The present article deals with the exothermic reactions model having constant heat source in the porous media with strong memory effects. The Caputo, Caputo-Fabrizio and Atangana-Baleanu fractional operators are used to induce memory effects in the mathematical modeling of exothermic reactions. The patterns of heat flow profiles are very essential for heat transfer in every kind of the thermal insulation. In the present investigation, we focus on the driving force problem due to the fact that temperature gradient is assumed. The mathematical equation of the problem is confined in a fractional energy balance equation (FEBE), which furnishes the temperature portrayal in conduction state having uniform heat source on steady state. The fractional Laplace decomposition technique is utilized to obtain the numerical solution of the corresponding FEBE describing the exothermic reactions. Some numerical results for the fractional exothermic reactions model are presented through graphs and tables.

Gangtao Liang, I. Mudawar

Abstract This paper provides a comprehensive review of published literature concerning heat transfer benefits of nanofluids for both macro-channels and micro-channels. Included are both experimental and numerical findings concerning several important performance parameters, including single-phase and two-phase heat transfer coefficients, pressure drop, and critical heat flux (CHF), each being evaluated based on postulated mechanisms responsible for any performance enhancement or deterioration. The study also addresses issues important to heat transfer performance, including entropy minimization, hybrid enhancement methodologies, and nanofluid stability, as well as the roles of Brownian diffusion and thermophoresis. Published results point to appreciable enhancement in single-phase heat transfer coefficient realized in entrance region, but the enhancement subsides downstream. And, while some point to the ability of nanofluids to increase CHF, they also emphasize that this increase is limited to short duration boiling tests. Overall, studies point to many important practical problems associated with implementation of nanofluids in cooling situations, including clustering, sedimentation, and precipitation of nanoparticles, clogging of flow passages, erosion to heating surface, transient heat transfer behavior, high cost and production difficulties, lack of quality assurance, and loss of nanofluid stability above a threshold temperature.

Gui-Lian Wang, Nan Qian, G. Ding

Abstract The heat transfer and flow characteristics of the microchannel heat sink (MCHS) with bidirectional ribs (BRs) are experimentally and numerically studied in the present paper. The BR, composed of vertical rib (VR) and spanwise rib (SR), can interrupt the thermal boundary layer and induce recirculation in both vertical and spanwise directions. Its cooling effectiveness is compared with that of the widely-used VR and SR for the Reynolds number ranged from 100 to 1000. The results show that the Nussalt number of the microchannel with BRs (BR-MC) is up to 1.4–2 and 1.2–1.42 times those of microchannels with VRs (VR-MC) and SRs (SR-MC), respectively. This implies that the BR can strengthen the heat transfer more sufficiently. Meanwhile, the utilizing of BR gives rise to the larger pressure drop penalty due to its broader obstruction areas. In addition, the higher relative rib height of VR (eVR) and relative rib width of SR (eSR) are revealed to enhance the heat transfer but induce pressure drop in the BR-MC. The thermal enhancement factor can keep larger than 1 when eVR

A. Baghban, M. Kahani, M. Nazari et al.

Abstract Nowadays, nanofluids are broadly utilized for various engineering and industrial systems including heat exchangers, power plants, air-conditioning, etc. The helically coiled tube heat exchangers are of the most interesting and efficient kinds of heat exchangers. The current study has focused on proposing model to predict Nusselt number by considering Prandtl number, volumetric concentration, and helical number of helically coiled heat exchanger as input variables. The investigated heat exchanger utilizes water carbon nanofluid. To propose an accurate model, a multilayer perceptron artificial neural network (MLP-ANN), adaptive neuro-fuzzy inference system (ANFIS), and least squares support vector machine (LSSVM) models are used. 72 experimental data are utilized as input data. Results indicate that LSSVM approach has the best performance and the proposed model by this approach has R-squared value equals to 1.