Countercurrent Double-Pipe Heat Exchanger Subjected to Flow-Rate Step Change,Part II: Analytical and Experimental Transient Response

This article investigates the analytical time constants of temperatures transient response along a countercurrent heat exchanger when a mass flow-rate step change is applied on hot fluid flowing through the inner duct. The time constants of the hot and cold fluids are spatially linear and the fluid not submitted to step change shows two types of transient response. The first corresponds to a linear spatial decreasing of the time constant, while the second presents a uniform time constant along the heat exchanger. For each case, the analytical expressions of time constants are derived taking the conditions of the transient response on the boundaries of the heat exchanger. The condition which enables distinguishing the two cases is also proposed in this article. The comparison between theoretical and experimental results allows us to validate the analytical expressions, which depend on the initial and final steady states. The influence of the magnitude of flow-rate step change on the transient behavior is studied by maintaining the initial steady state. In the same way, the initial steady-state effect is investigated with the same magnitude of the flow-rate step change. The comparison of response time to positive and negative flow-rate step change is also presented for different values of cold-fluid flow rate.     

Countercurrent Heat Exchangers with Both Fluids Subjected to External Heating

A mathematical model was developed to study the performance of a counterflow heat exchanger in which both fluid streams are exposed to external heating. It was found that under the external heat transfer condition, the effectiveness of the heat exchanger was drastically reduced. The hot fluid temperature effectiveness increased as the external thermal conductance ratios, Rh and Rc, decreased. For effective operation and to avoid temperature cross, there exist a maximum possible NTU for a given Rh and Rc.     

Transient Response of a Serpentine Finned-Tube Cross-Flow Heat Exchanger to a Step Change in Inlet Temperature

Abstract

An analysis of the thermal response of a finned-tube, liquid-to-gas cross-flow heat exchanger due to a step change in the liquid inlet temperature is performed. Closed-form solutions for the liquid and gas temperatures as functions of space and time are obtained via the Laplace transform technique for both small and large arguments of the modified Bessel function of the first kind. Using four physically important dimensionless parameters, the response of the liquid and average gas outlet temperatures are studied and presented in the time domain. The analysts is extended to a single-row serpentine coil geometry by accounting for U-tube bends. Using a typical heat exchanger geometry, the effects of the tube bends are shown to be significant. Relevant applications include automotive and HVAC heat exchangers and systems.     

热负荷阶跃变化条件下热回流式换热系统缺陷机理及优化方法研究

针对国内某北方核电厂的设备冷却水系统热回流式换热器,分析了不同热负荷下热回流式换热系统的稳态特性及负荷阶跃变化下热回流式换热系统缺陷机理,提出了热回流式换热器系统优化方法.研究表明:在不同热负荷下热回流式换热器系统切换的关键是与不同热负荷对应的具有特定温度的伴流的形成;热负荷阶跃变化下状态转换瞬态过程中存在系统缺陷,其...     

相变-液浴式新型换热器的理论研究

提出了一种新型的中间热媒式换热器———相变-液浴式换热器,并介绍了它的工作原理。通过分析指出,该换热器除了具有可将冷、热流体严格隔离的特点外,还具有一个十分重要的特性,即:可以在换热量大幅度改变时,保持换热室内的压力和温度基本不变。这些特性使其有可能在锅炉、工业余热回收等方面发挥很大作用。图2参7     

The Cartridge Heat Exchanger: A New Concept for Difficult Services

A new concept in the design of heat exchangers has been developed and is now in production. This paper outlines the generic design concept of the cartridge heat exchanger.     

Helical Baffles in Shell-and-Tube Heat Exchangers,Part I: Experimental Verification

Performance of heat exchangers with helical baffles, or helixchangers, is discussed using the results of tests conducted on units with uarious baffle geometries. An optimum helix angle is identified at which the conversion efficiency for converting pressure drop to heat transfer on the shell side of helixchangers is maximized. Designs for standard industry applications are optimized using the analysis of test results.     

Condensation in Horizontal Smooth Tubes: A New Heat Transfer Model for Heat Exchanger Design

This paper proposes a new method to determine the condensation heat transfer coefficient of fluids flowing into horizontal smooth tubes with internal diameters D > 3 mm. The method has been drawn up as simply as possible and is ready to use in heat exchanger modeling and design applications. It is also suitable to work very well with old and new fluids used in the refrigeration, air conditioning, and heat pump industries. Particular attention is given to accuracy: it has been tested over a wide updated experimental database and comes from many different independent researchers with reduced experimental uncertainties. In order to obtain an easy structure, only two equations are employed, related respectively to & Delta; T-independent and to & Delta; T-dependent fluid flows. All the parameters that influence the condensation heat transfer have been included.

A comparison has been conducted against HCFCs, HFCs, HCs, carbon dioxide, ammonia, and water data. Zeotropic mixtures with two and three components are also considered in the comparison by applying the Bell and Ghaly [1 Bell, K. J. and Ghaly, M. A. 1973. An Approximate Generalized Design Method for Multicomponent/Partial Condenser. AIChE Symp. Ser., vol. 69: 72–79. [CSA] [Google Scholar]] correction to calculate the relative heat transfer penalization. A model has been developed with the idea of getting high accuracy through an easy structure, and the results show a very satisfactory agreement with experimental data: average deviation e_R = +2%, absolute mean deviation e_AB = 14%, and standard deviation σ_N = 19% for the total number of 5478 data points.     

Ultrafast Laser-Induced Elastodynamics in Single Crystalline Silicon Part I: Model Formulation

The various responses of a silicon wafer excited by a femtosecond pulsed laser are investigated. A multi-time scale axisymmetric model that governs the transport dynamics in silicon is presented based on the relaxation-time approximation of the Boltzmann equation. Temperature-dependent multi-phonons, free-carrier absorptions, and the recombination and impact ionization processes are considered using a set of balance equations. The mechanical response of the lattice is described by momentum equations. To solve the model of 17 coupled time-dependent partial differential equations without having to be concerned with non-physical oscillations in the solution, an implicit finite difference scheme on a staggered grid is developed. The staggered finite difference scheme allows velocities and first-order spatial derivative terms to be calculated at locations midway between two consecutive grid points, and shear stresses to be evaluated at the center of each element. A multi-time-scale approach involving the use of varying time steps ranging from 5 fs to 5 ps is implemented to successfully obtain time integration results up to 10 ns.     

Optimization of Three-Phase Induction Motor Design Part I: Formulation of the Optimization Technique

This two-part paper deals with the optimization of the induction motor designs with respect to cost and efficiency. Most studies on the design of an induction motor using optimization techniques are concerned with the minimization of the motor cost and describe the optimization technique that was employed, giving the results of a single (or several) optimal design(s). In the present paper, a more comprehensive study on the optimization of a three-phase induction motor design was performed. This includes the relationship between motor cost, efficiency, and power factor; the effect of the properties of the electrical steel; and other effects as they occur in an optimal design. In addition, the optimization procedure that was used in this paper includes a design program, where some of the secondary parameters (which are called here variable constants), are modified according to the optimal results, in contrast to other studies where these parameters remain constant for the entire optimization. In this part, a new mathematical formulation of the optimization problem of the induction motor is presented.     

The distributed-energy chain model for rapid coal devolatilization kinetics. Part I: Formulation

The distributed-energy chain model (DISCHAIN) interprets coal devolatilization in terms of independent influences from chemical reaction rates and from macromolecular configuration. Coal is represented by three components: (1) aromatic units that are attached pairwise by (2) labile bridges to form nominally infinite linear chains, with (3) peripheral groups branching from the aromatic units. These components are the building blocks for unreacted coal, free monomers (mobile aromatic units), gas, tar, and char. Four chemical reactions represent bridge dissociation, peripheral group elimination, and tar and char formation. Analytic probability expressions and competitive reactions describe the conversion of bound aromatic units into free monomers, and enter into the formation of all products except gas. There are no hypothetical ultimate yields.The model is introduced in two parts. Here in Part I, the coal model, chemical reactions, and chain statistics are derived and formulated into rate equations. Mechanisms leading to major products are identified, including a novel mechanism for yield enhancement by faster heating. Whenever bridge dissociation and char formation occur concurrently, as for slow heating, the subsequent generation of monomers is inhibited. Aromatic units are thereby excluded from the competition between tar and char formation. Conversely, bridge dissociation and char formation occur consecutively for rapid heating, and a greater proportion of the original bound aromatic units become monomers and, ultimately, tar.     

Shell-and-Tube Heat Exchanger Geometry Modification: An Efficient Way to Mitigate Fouling

Abstract

Crude oil fouling of a shell-and-tube heat exchanger sized according to TEMA standard is compared to a No-Foul design under industrial operating conditions. For similar operating conditions, TEMA and No-Foul heat exchangers have the same behavior regarding fouling. Since the No-Foul one has less tubes by design for the same heat duty, shear stress is increased. Consequently, the No-Foul heat exchanger is less prone to fouling at the same throughput. Impact of tube bundle geometry is then investigated. Helically finned tubes are compared to plain tubes in the No-Foul heat exchanger. Under similar operating conditions, fouling rates measured are up to an order of magnitude lower than plain tubes (respectively 10^?11 and 10^?10 m² K/J). However, pressure drop across the tube-side in both No-Foul plain and finned setup are increased in comparison to the TEMA heat-exchanger.     

A Risk-Based Performance Analysis of Plate-andFrame Heat Exchangers Subject to Fouling: Economics of Heat Exchanger Cleaning

Fouling is one of the major uncertainties associated with the operation and maintenance of plate-and-frame heat exchangers (PHEs) in the steel and process industries. The decision regarding periodic maintenance (i.e., cleaning) to meet the target performance level is generally based on both thermal and economic behavior of the process. In this paper, we present a cost model that includes the risk level and the scatter parameter of random fouling growth models. Two models (namely, power law and exponential fouling growth) are integrated in the model. The non-dimensional cost function of reduced time is examined by considering dimensionless cost parameters, representing additional cost due to a decrease in effectiveness, pumping power cost, anti-foulant cost, cleaning cost, additional pumping power cost of the standby unit during cleaning, and miscellaneous costs. These dimensionless cost elements are examined for a PHE operating in the steel industry. The results are presented in terms of the risk level and scatter parameter for the underlying fouling models. Furthermore, a simplified closed-form solution is also obtained to study the optimal cycle time, which represents the minimum cost of operation and maintenance of heat exchangers. It is found that the optimum dimensionless cost increased by about 21 and 14% for the power law and exponential fouling models, respectively, as the risk level decreased from 0.5 to 0.01     

Heat transfer enhancement in dense suspensions of agitated solids. Part I: Theory

We predict heat transfer enhancement through dense homogeneous suspensions of agitated solids in conductive fluids by coupling the fluid and solid phases through a volumetric source term. The enhancement is governed by a Damköhler number demarcating an “exchange limit” where the source term dominates, and a “diffusion limit” set by the ability of agitated particles to self-diffuse. We point out effects of particle ordering on mixture conductivity and volumetric heat exchange rate, carry out thermal simulations to justify the form of these terms, and model further enhancements from gas velocity fluctuations induced by solids of high agitation.     

Heat and mass transfer in gas metal arc welding. Part I: The arc

A unified comprehensive model was developed to simulate the transport phenomena occurring during the gas metal arc welding process. An interactive coupling between arc plasma; melting of the electrode; droplet formation, detachment, transfer, and impingement onto the workpiece; and weld pool dynamics all were considered. Based on the unified model, a thorough investigation of the plasma arc characteristics during the gas metal arc welding process was conducted. It was found that the droplet transfer and the deformed weld pool surface have significant effects on the transient distributions of current density, arc temperature and arc pressure, which were normally assumed to be constant Gaussian profiles.     

Use of Chilton-Colburn Analogy to Estimate Effective Plume Chimney Height of a Forced Draft,Air-Cooled Heat Exchanger

An estimation of the effective plume-chimney height above a forced draft, air-cooled heat exchanger operating under natural convection has been carried out by employing Chilton-Colburn and Reynolds analogies of natural convection heat transfer on a flat plate to calculate the differential pressure in driving entrained air from the surrounding air. The results show that the Chilton-Colburn analogy performs closely with the Nusselt number–Equivalence method previously used by Chu [1 Chu, C. M. 2002. A Preliminary Method for Estimating the Effective Plume Chimney Height above a Forced-Draft Air-Cooled Heat Exchanger Operating under Natural Convection. Heat Transfer Engineering, vol. 23(no. 3): 3–12. [CSA][Taylor & Francis Online], [Web of Science ®] , [Google Scholar]] and renders support for its applicability.     

Strained flamelets for turbulent premixed flames, I: Formulation and planar flame results

A strained flamelet model is proposed for turbulent premixed flames using scalar dissipation rate as a parameter. The scalar dissipation rate of reaction progress variable is a suitable quantity to describe the flamelet structure since it is governed by convection-diffusion-reaction balance and it is defined at every location in the flamelets, which are represented by laminar flames in reactant-to-product opposed flow configuration. The mean reaction rate is obtained by using the flamelets reaction rate and the joint pdf of the progress variable and its dissipation rate. The marginal pdf of the progress variable is presumed to be β-pdf and the pdf of the conditional dissipation rate is taken to be log-normal. The conditional mean dissipation rate is obtained from modelled mean dissipation rate. This reaction rate closure is assessed using RANS calculations of statistically planar flames in the corrugated flamelets and thin reaction zones regimes. The flame speeds calculated using this closure are close to the experimental data of Abdel-Gayed et al. (1987) [27] for flames in both the regimes. Comparisons with other reaction rate closures showed the benefits of the strained flamelets approach.     

Evaluation of alternative thermochemical cycles,Part I: The methodology

The Nuclear Hydrogen Initiative (NHI) of the U.S. Department of Energy′s Office of Nuclear Energy Science and Technology is supporting an effort to reevaluate thermochemical cycles reported in the literature as having both promising efficiencies and proof-of-concept results. Recognizing that the calculation of efficiencies was not always consistently done or well defined in the literature, we first developed a consistent methodology for reevaluating the candidate thermochemical cycles. This methodology was defined for three levels of maturity in process knowledge. Argonne National Laboratory and a group of universities recalculated the efficiency for each level and identified the most critical R&D necessary to further assess the cycles' potential. This methodology is illustrated with the Cu–Cl in Part I of this series of three papers. Current results of the analyses for all of the cycles are summarized in Part II. Part III contains a more detailed Level 3 analysis for the Cu–Cl cycle.     

High-pressure Condensing Refrigerant Flows Through Microchannels,Part II: Heat Transfer Models

ABSTRACT

Condensation of high-pressure refrigerants in small-diameter channels over a wide range of reduced pressures approaching the critical point is investigated in this two-part study. Part I presented pressure drop measurements and a two-phase pressure drop model. In this paper, Part II of the study, a condensation heat transfer model is presented. Heat transfer coefficients were measured during condensation of R404A in circular channels (inner diameter = 0.86, 1.55, 3.05 mm) over the entire quality range. The saturation temperature was varied from 30 to 60°C, and mass flux from 200 to 800 kg m^-2 s^-1, to evaluate their effects on condensation heat transfer coefficient. The heat transfer model is developed using a microchannel flow regime map and the void fraction model previously developed by the authors. The resulting model predicts 93.6% of the data within ±25%. The model exhibited good agreement with data from condensing ammonia and carbon dioxide, predicting 84.8% and 97% of their data within ±25%, respectively.