The Influence of Core Capacitance on the Dynamic Performance of a Single-Phase Natural Circulation Loop With End Heat Exchangers

The effect of wall-core capacitance of heat exchangers on the dynamic behavior of a natural circulation loop (NCL) with end heat exchangers is studied under various excitations such as step, ramp, exponential, and sinusoidal. The transient one-dimensional conservation equations are derived for loop fluid, hot and cold fluid streams, and wall core of both heat exchangers. The solution of a set of transient partial differential equations and one integro-differential equation for loop fluid circulation rate is achieved through a finite-element technique. Imposing the excitations to the inlet temperature of hot fluid, the effects of wall-core capacitance on the responses of outlet temperatures of both hot and cold fluid streams and flow rate of loop fluid are studied. Wall-core capacitance diminishes the initial transients and delays the inception of hot and cold fluids outlet temperature profiles as well as loop fluid flow profile. Further, it has the ability to bring even unstable system behavior with reverse flows into a stable system with steady loop flow rate through quickly decaying oscillations. System responses are also greatly influenced by boundary conditions such as hot and cold fluids flow rates and their inlet temperature excitations such as step, ramp, and exponential. As flow stability is an important subject for single-phase NCLs, a stability map is constructed and compared with zero wall-core capacitance. Inclusion of wall-core capacitance in the present study reveals the important fact that the stable state operating zone widens with the wall-core capacitance.     

Parallel-Flow Heat Exchangers with Ambient Thermal Interaction

A mathematical model is developed to study the performance of a parallel-flow heat exchanger in which both fluid streams are interacting thermally with the surroundings. The fluid temperatures are found to be dependent on the magnitude of the ambient temperature relative to fluid inlet temperatures, the ratios of conductances between the fluids and the ambient and the interfluid conductance, the ratio of minimum to maximum fluid capacities, and the number of transfer units, NTU, for the heat exchanger. Two heat exchanger effectiveness criteria, one each for the hot and cold fluids, are used to study performance. The effectiveness is found to be adversely affected by increasing conductance ratios, increasing NTU, and increasing temperature difference between the ambient and the fluid of interest. For very high values of the conductance ratios, the heat exchanger will not perform as expected and both fluid temperatures will approach that of the ambient. The parallel-flow arrangement is compared to counterflow and is found to be less effective under the external heat transfer condition.     

Performance Optimization of Co-current Gas-to-Gas Micro Heat Exchanger Considering Radiation

Optimization of a parallel flow gas-to-gas tubular micro heat exchanger with hot core and cold annulus fluid is numerically analyzed, considering the beneficial role of surface radiation. Operating and geometric parameters are varied for fixed overall mass flow rate and temperature of cold core fluid, to study the effects on the following performance parameters: heat transferred to annulus fluid, logarithmic mean temperature difference, effectiveness, and volumetric heat transfer coefficient. The micro heat exchanger is optimized for high heat transfer to annulus fluid and volumetric heat transfer coefficient, for different operating and geometric conditions. Optimization for high volumetric heat transfer coefficient maximizes the micro heat exchanger effectiveness, heat transferred and improves logarithmic mean temperature difference.     

A new methodology for simultaneous optimization of capital and operating cost targets in heat exchanger network design

A new approach has been developed for establishing cost-optimal targets for a heat exchanger network (HEN). We formulate a mixed-integer-linear-programming (MILP) transportation problem that simultaneously optimizes for heat exchanger units, heat exchange area and loads on each utility. Heat exchange matches are placed between each hot stream in each temperature interval and all cold streams in all the subsequent lower temperature intervals. The objective function is linear and minimizes the total annual cost of the HEN subject to heat balance constraints. The temperature intervals are generated with a temperature shift just greater than zero. Each temperature interval is small enough that we can linearize the log-mean temperature difference on each match while maintaining the accuracy. Flowrate continuity constraints are written on utility streams so that we could account for non-point utilities. Furthermore, constraints on heat exchange area that occur in retrofit scenarios are incorporated into the model. The solution of the optimization model provides the heat loads on each utility, in case of multiple hot and cold utilities, the heat load on every match pair, the heat exchange area target and the number of units target such that the total annual cost is minimum. Using these targets, it is straightforward to construct heat exchanger networks.     

Cooling load density characteristics of an endoreversible variable‐temperature heat reservoir air refrigerator

The performance optimization of an endoreversible air refrigerator with variable‐temperature heat reservoirs is carried out by taking the cooling load density, i.e. the ratio of cooling load density to the maximum specific volume in the cycle, as the optimization objective in this paper. The analytical relations of cooling load, cooling load density and coefficient of performance are derived with the heat resistance losses in the hot‐ and cold‐side heat exchangers. The maximum cooling load density optimization is performed by searching the optimum pressure ratio of the compressor, the optimum distribution of heat conductance of the hot‐ and cold‐side heat exchangers for the fixed total heat exchanger inventory, and the heat capacity rate matching between the working fluid and the heat reservoirs. The influences of some design parameters, including the heat capacitance rate of the working fluid, the inlet temperature ratio of heat reservoirs and the total heat exchanger inventory on the maximum cooling load density, the optimum heat conductance distribution, the optimum pressure ratio and the heat capacity rate matching between the working fluid and the heat reservoirs are provided by numerical examples. The refrigeration plant design with optimization leads to a smaller size including the compressor, expander and the hot‐ and cold‐side heat exchangers. Copyright © 2002 John Wiley & Sons, Ltd.     

线性拟合换热网络性能与进口温度的关系

换热网络问题属于典型的混合整数非线性问题,有严重的非凸、非线性特性,且影响换热网络性能的因素众多.主要研究了换热网络进口温度与性能之间的关系.给定一换热网络固定结构,同时在固定面积的条件下,变动每股流体的进口温度,分析流体的进口温度与换热网络性能之间的关系,得出线性拟合的适用性.然后设计正交试验,拟合出性能与进口温度的线性模型,并验证该模型的准确性.最后根据拟合模型,从众多流股中优选冷、热流体组合成目标换热网络.     

Effectiveness–NTU Relations for Counterflow Heat Exchangers: The Effect of Kinetic Energy Variation and Heat Leak From Outside

The effectiveness–number of transfer units (NTU) relations are useful data for designing and performance evaluation of heat exchangers with fluids having considerable variation in velocities in the presence of heat leak. In this article, the closed-form (benchmark) solutions for counterflow heat exchangers, when the heat leak is either on the hot or cold side of the heat exchanger in the presence of kinetic energy variation, are presented. It was found that the effectiveness depends on NTU and fluid capacity ratio along with six other dimensionless variables that reflect the magnitude and axial distribution of the kinetic energy and heat leak on the hot and cold sides of the heat exchanger. The results are also presented in a graphical form exhibiting the variation of effectiveness of the heat exchanger with the already-mentioned parameters. It was demonstrated that when the dimensionless heat leak and kinetic energy terms approach zero, the solution reduces to the classical effectiveness–NTU relations for counterflow heat exchangers.     

Thermal performance and pressure drop of the helical-coil heat exchangers with and without helically crimped fins

In the present study, the thermal performance and pressure drop of the helical-coil heat exchanger with and without helical crimped fins are studied. The heat exchanger consists of a shell and helically coiled tube unit with two different coil diameters. Each coil is fabricated by bending a 9.50 mm diameter straight copper tube into a helical-coil tube of thirteen turns. Cold and hot water are used as working fluids in shell side and tube side, respectively. The experiments are done at the cold and hot water mass flow rates ranging between 0.10 and 0.22 kg/s, and between 0.02 and 0.12 kg/s, respectively. The inlet temperatures of cold and hot water are between 15 and 25 °C, and between 35 and 45 °C, respectively. The cold water entering the heat exchanger at the outer channel flows across the helical tube and flows out at the inner channel. The hot water enters the heat exchanger at the inner helical-coil tube and flows along the helical tube. The effects of the inlet conditions of both working fluids flowing through the test section on the heat transfer characteristics are discussed.     

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.     

Effect of ambient heat-in-leak on the performance of a three fluid heat exchanger,for cryogenic applications,using finite element method

In most cryogenic applications, heat in leak from the ambient is a significant factor for the degradation in the performance of heat exchangers. The effect of heat in leak to the cold fluid in a three-fluid heat exchanger, for a cryogenic application, involving thermal interaction between all the three fluids, has been investigated using both the analytical and finite element methods. Cooling of the hot fluid has been identified as the objective of the three fluid heat exchanger. Seven non-dimensional parameters, including one to account for ambient heat in leak to the cold fluid, have been identified and their effects on hot fluid behaviour – temperature profile, effectiveness and degradation factor – have been studied. The results presented give valuable inputs towards better understanding of the behaviour of the hot fluid in this class of heat exchangers.     

Numerical and Experimental Investigation of Heat Transfer of α-Al2O3/Water Nanofluid in Double Pipe and Shell and Tube Heat Exchangers

This research presents an experimental and numerical study on the heat transfer of α-Al₂O₃/water nanofluid flowing through the double pipe and shell and tube heat exchangers, under laminar flow conditions. Effects of important parameters such as hot and cold volume flow rates, nanofluid temperature, and nanoparticles concentration on the heat transfer characteristics are investigated. The results indicated that the heat transfer performance of both double pipe and shell and tube heat exchangers increases with increasing the hot and cold volume flow rates, as well as the particle concentrations and nanofluid inlet temperature. Compared with pure water, the results indicated that the heat transfer coefficients of nanofluid in the double pipe and shell and tube heat exchangers are higher than those of water by 13.2% and 21.3%, respectively. Also, the heat transfer performance of nanofluid in a shell and tube heat exchanger is 26.2% higher than the double pipe heat exchanger. A computational fluid dynamics (CFD) technique was used for heat transfer simulation in the previously mentioned heat exchangers. Computed overall heat transfer coefficients of the nanofluids are in good agreement with the experimental data.     

Experimental Performance of R-134a-Filled and Water-Filled Loop Heat Pipe Heat Exchangers

Experimental investigations were conducted to determine the thermal performances of an R-134a-filled thermosyphon heat pipe heat exchanger (THPHE) and a water-filled loop heat pipe heat exchanger (LHPHE) for hot and cold energy recovery for air conditioning purposes. For such applications, the heat pipe heat exchangers are operated at low temperatures. Both exchangers were operated in the countercurrent flow mode. This article presents the experimental results obtained. The results showed that heat transfer rate increased as evaporator inlet temperature increased and as both evaporator and condenser velocities increased. The overall effectiveness for the THPHE ranged from 0.8 to a minimum of about 0.5, while for the LHPHE it ranged from 0.9 to 0.3. Overall effectiveness was found to approach a minimum when both air streams have equal velocities.     

Countercurrent Double-Pipe Heat Exchanger Subjected to Flow-Rate Step Change,Part I: New Steady-State Formulation

This article deals with a new steady-state formulation of temperatures along a double-pipe heat exchanger in counterflow configuration when the mass flow rate is submitted to step change. The steady-state method is based on estimation of the exponential factor of temperature profile. This method, compared to results obtained from correlations, gives an alternative approach to the well-known procedures for sizing and rating heat exchangers in industrial applications, such as the k -NTU and LMTD methods. It is based on the average convective heat transfer coefficients of the hot and cold sides, while the other methods are based on the overall heat transfer coefficient.     

Dendritic constructal heat exchanger with small-scale crossflows and larger-scales counterflows

This paper describes the constructal route to the conceptual design of a two-stream heat exchanger with maximal heat transfer rate per unit volume. The flow structure has multiple scales. The smallest (elemental) scale consists of parallel-plates channels the length of which matches the thermal entrance length of the small stream that flows through the channel. This feature has two advantages: it eliminates the longitudinal temperature increase (flow thermal resistance) that would occur in fully developed laminar flow, and it doubles the heat transfer coefficient associated with fully developed laminar flow. The elemental channels of hot fluid are placed in crossflow with elemental channels of cold fluid. The elemental channel pairs are assembled into sequentially larger flow structures (first construct, second construct, etc.), which have the purpose of installing (spreading) the elemental heat transfer as uniformly as possible throughout the heat exchanger volume. At length scales greater than the elemental, the streams of hot and cold fluid are arranged in counterflow. Each stream bathes the heat exchanger volume as two trees joined canopy to canopy. One tree spreads the stream throughout the volume (like a river delta), while the other tree collects the same stream (like a river basin). It is shown that the spacings of the elemental and first-construct channels can be optimized such that the overall pumping power required by the construct is minimal. The paper concludes with a discussion of the advantages of the proposed tree-like (vascularized) heat exchanger structure over the use of parallel small-scale channels with fully developed laminar flow.     

Counter-current tubular heat exchanger: Modeling and adaptive predictive functional control

The control of the outlet temperature of a counter-current tubular heat exchanger in heater configuration with the predictive functional control is presented in this paper. The outlet temperature of the cold fluid is controlled by variation of the flow of the hot fluid while the inlet temperatures corresponding to the principal inputs are maintained constant. An approximated first order model, corresponding to the response of the heat exchanger to a step change of the flow rate is used to apply the functional predictive control. The gain and the time-constant of this model depend on the initial and final steady state temperatures according to the flow rates. This nonlinear dynamic model, obtained from the partial differential equations (PDE) is taken into account to apply the functional predictive control, which was validated experimentally in various configurations. The robustness of this controller is also examined when the system is subjected to the sudden change of the flow rate of the cold fluid.     

板式换热器性能的数值模拟

建立了人字形板式换热器冷热双流道的流体流动与传热计算模型,利用计算流体力学软件对5组不同速度工况下换热器内流体的流动和传热进行了数值模拟,分析了换热器流道内的速度场、温度场和压力场.结果表明:数值模拟得到的板式换热器进、出口温差和压降与试验测量值的误差均小于6%;换热器内流体的流动和传热存在明显的不均匀性,导致其进、出口的另一侧出现明显的传热"死区";换热器的总传热系数和流道阻力均随着流体流速的增大而增大.     

浮升力对T型管道内冷热流体混合过程热波动的影响

利用Fluent软件对T型管道内冷热流体混合过程进行了大涡模拟,考虑了温差引起的浮升力,获得了混合过程的瞬时温度和瞬时速度,通过定义时均值和均方根值来描述温度和速度的平均大小和波动程度.数值模拟结果表明,理查德森数对冷热流体的混合有着重要的影响:当理查德森数为负值且绝对值越大,则温度和速度的波动也就越大;相反,当理查德...     

Heat Exchangers Design Under Variable Overall Heat Transfer Coefficient: Improved Analytical and Numerical Approaches

In typical heat exchanger design methods it is generally assumed that the overall heat transfer coefficient is constant and uniform; however, the heat transfer coefficients on the hot and cold sides of the heat exchanger may vary with flow Reynolds number, surface geometries, fluid thermophysical properties, and other factors. In this article we present simple analytical and numerical methods for calculating heat transfer area for data sets introduced earlier in the literature. For the analytical methods presented in the article, the variation in the overall heat transfer coefficient with the local hot and cold fluid temperature difference is expressed as a power-law model and as a general polynomial model. The procedure for calculating the heat transfer area with the power-law model is explained with respect to a simple closed-form solution, while the polynomial model can also provide an analytical solution that seems to be quite accurate for the data sets examined. It is also shown that a Chebyshev numerical integration scheme that requires four points compared to the Simpson method of three points is quite accurate (within 1% of the exact value).     

Perfluoropolyethers Coatings Design for Fouling Reduction on Heat Transfer Stainless-Steel Surfaces

The scope of this research is to obtain a film coating on stainless-steel surfaces in order to reduce the interaction between the metal surface and the precipitates, so as to mitigate fouling in heat exchangers. Perfuoropolyethers were used to obtain nano-range fluorinated layers in order to make hydrophobic the stainless-steel surfaces. A pilot plant with two identical heat exchangers was built to investigate the ability of the hydrophobic coating of preventing fouling. The heat exchangers, installed in parallel, operated at the same temperature and pressure conditions, namely, laminar flow regime and inlet flow temperatures of 291–293 K for cold streams and 313–333 K for hot streams. We compared the heat transfer performance of the two heat exchangers. After a 5-month operation, the decrease in the heat transferred was 56% for the coated heat exchanger and 62% for the uncoated heat exchanger. Moreover, the increase of heat transfer resistance due to scale on the uncoated heat exchanger, with respect to the coated one, was three times higher.     

Theoretical prediction of longitudinal heat conduction effect in cross-corrugated heat exchanger

In the elementary heat exchanger design theory, the longitudinal heat conduction through the heat transfer plate separating hot and cold fluid streams is neglected, and only the transverse heat conduction is taken into account for the conjugate heat transfer problem. In the cross-corrugated heat exchanger, the corrugated primary surface naturally leads to the highly non-uniform convective heat transfer coefficient distribution on opposite sides of the plate. In such a case, the longitudinal heat conduction may play a significant role in the thermal coupling between high heat transfer regions located on opposite sides of the plate. In the present study CFD is used to perform a quantitative assessment of the thermal performance of a cross-corrugated heat exchanger including the longitudinal heat conduction effect for various design options such as different plate thickness and corrugation geometry for a typical operating condition. The longitudinal heat conduction effect is then predicted by the theoretical method using the ‘network-of-resistance’ in the wide range of the heat exchanger design space.