Cold Heat Exchanger Research Articles

Dynamic performance of a natural circulation loop with end heat exchangers under different excitations

In the present work the dynamic performance of a natural circulation loop (NCL) has been studied under step, ramp, exponential and sinusoidal excitations. The loop is equipped with two heat exchangers at its lower and upper end for the heating and cooling of the loop fluid. For the analysis, transient one-dimensional conservation equations have been constructed for the loop fluid as well as for the two fluid streams of hot and cold end heat exchangers. The solution of a set of differential equations and one integro-differential equation has been obtained through a finite element method (FEM). For different excitations imposed to the inlet temperature of the hot fluid responses have been studied for the outlet temperature of the two fluid streams and the mass flow rate of the coupling fluid. It has been observed that all these quantities experience some initial transients before reaching the steady state. Time needed for the attainment of steady state varies with the type of excitation. A finite time delay is observed before the cold fluid stream temperature starts responding to the excitation. This delay is related to the time required for the advection of a fluid particle.

Performance analysis and comparison of an Atkinson cycle coupled to variable temperature heat reservoirs under maximum power and maximum power density conditions

In this paper, performance analysis and comparison based on the maximum power and maximum power density conditions have been conducted for an Atkinson cycle coupled to variable temperature heat reservoirs. The Atkinson cycle is internally reversible but externally irreversible, since there is external irreversibility of heat transfer during the processes of constant volume heat addition and constant pressure heat rejection. This study is based purely on classical thermodynamic analysis methodology. It should be especially emphasized that all the results and conclusions are based on classical thermodynamics. The power density, defined as the ratio of power output to maximum specific volume in the cycle, is taken as the optimization objective because it considers the effects of engine size as related to investment cost. The results show that an engine design based on maximum power density with constant effectiveness of the hot and cold side heat exchangers or constant inlet temperature ratio of the heat reservoirs will have smaller size but higher efficiency, compression ratio, expansion ratio and maximum temperature than one based on maximum power. From the view points of engine size and thermal efficiency, an engine design based on maximum power density is better than one based on maximum power conditions. However, due to the higher compression ratio and maximum temperature in the cycle, an engine design based on maximum power density conditions requires tougher materials for engine construction than one based on maximum power conditions.

Stability Behavior of a Natural Circulation Loop with End Heat Exchangers

The present investigation describes the stability behavior of NCL with end heat exchangers. The one-dimensional transient conservation equations of the loop fluid and the two fluid streams of cold end and hot end heat exchangers are solved simultaneously using the finite element program. For the stability analysis the loop response is found for an imposed finite perturbation to the loop circulation rate. Though the stability may depend on the number of parameters, variation of two nondimensional parameters, namely Ch* and GrL, is studied. Selecting the specific combinations of the above two parameters three different cases of stability, namely, stable, neutrally stable, and unstable, are demonstrated. The stability behavior is scanned over a wide range of Ch* and GrL values and the stability envelope is also constructed.

Theoretical study on a Miniature Joule–Thomson & Bernoulli Cryocooler

In this paper, a microchannel-based cryocooler consisting of a compressor, a recuperator and a cold heat exchanger has been developed to study the feasibility of cryogenic cooling by the use of Joule–Thomson effect and Bernoulli effect. A set of governing equations including Bernoulli equations and energy equations are introduced and the performance of the cooler is calculated. The influences of some working conditions and structure parameters on the performance of coolers are discussed in details.

The influence of gravity on Rayleigh streaming in thermoacoustic systems

The thermal buffer tube is the component of thermoacoustic systems intended to transmit sound and to provide a thermal barrier between the hot and cold heat exchangers located at the opposite ends of the tube. In high-power operating regimes of thermoacoustic devices, the sound amplitude is high enough to cause significant mass (Rayleigh) streaming in the thermal buffer tube. The streaming flow convects heat, which can degrade the overall performance of a thermoacoustic system by thermally short-circuiting the hot and cold heat exchangers. This leads to increased heat consumption by prime movers and to reduced cooling capacity in refrigerators. The thermal buffer tube is characterized by a significant axial variation of temperature, so effects of gravity on the streaming should be important. A simple analytical model allows quantitative estimation of gravity effects, showing that gravity may significantly suppress the streaming under certain conditions. The important parameters of the system and operating conditions that affect the streaming in the presence of gravity are identified. [Work supported by DOE’s Office of Science.]

Response of a thermoacoustic refrigerator to the variation of the driving frequency and loading

The performance of a thermoacoustic refrigerator subjected to variable loading was analyzed experimentally and the data were compared with those obtained using a computational model. The computational model relies on one-dimensional cross-section-averaged equations discretized using the network analogy. The thermoacoustic refrigerator was modeled by dividing it into 1 mm long slices in the direction of the acoustic axis. The hot heat exchanger of the thermoacoustic refrigerator was maintained at ambient temperature and the temperature of the cold heat exchanger was varied to achieve temperature differences of ΔT=0, 5 and 10 K along the stack. The cooling load was measured and calculated for these temperature differences while varying the driving frequency between 30 and 65 Hz. The contribution of the progressive and stationary waves and the losses on the thermoacoustic heat flow was computed and discussed.

Regenerator-based thermoacoustic refrigerator for ice cream storage applications

A regenerator-based chiller has been built in the ‘‘bellows bounce’’ style [J. Acoust. Soc. Am. 112, 15 (2002)] to replace the vapor compression system in an ice cream sales cabinet. It utilizes a 6-in.-diam metal bellows to form a compliant cavity that contains the dynamic pressure oscillation (>50 kPa). The stiffness of the gas trapped in the bellows is resonated against the mass of the bellows-cap and the mass of a moving-magnet linear motor which is capable of high (>85%) electro-acoustic efficiency. A second resonator, operated well below its natural frequency, uses the gas stiffness of a 1-l volume nested within the bellows and the inertia of an ordinary loudspeaker cone to create the pressure difference across the regenerator that drives gas flow that is in-phase with pressure. The mass of the cone can be adjusted to vary the multiplication factor that is typically 5%–10% greater than the dynamic pressure within the bellows. The loudspeaker cone suffers none of the hydrodynamic losses associated with an acoustic inertance and eliminates problems with dc gas flow in the energy feedback path. The cold heat exchanger forms one surface of the pressure vessel permitting direct contact with any thermal load. [Work supported by Ben and Jerry’s Homemade.]

Performance analysis for an irreversible variable temperature heat reservoir closed intercooled regenerated Brayton cycle

In this paper, the theory of finite time thermodynamics is used in the performance analysis of an irreversible closed intercooled regenerated Brayton cycle coupled to variable temperature heat reservoirs. The analytical formulae for dimensionless power and efficiency, as functions of the total pressure ratio, the intercooling pressure ratio, the component (regenerator, intercooler, hot and cold side heat exchangers) effectivenesses, the compressor and turbine efficiencies and the thermal capacity rates of the working fluid and the heat reservoirs, the pressure recovery coefficients, the heat reservoir inlet temperature ratio, and the cooling fluid in the intercooler and the cold side heat reservoir inlet temperature ratio, are derived. The intercooling pressure ratio is optimized for optimal power and optimal efficiency, respectively. The effects of component (regenerator, intercooler and hot and cold side heat exchangers) effectivenesses, the compressor and turbine efficiencies, the pressure recovery coefficients, the heat reservoir inlet temperature ratio and the cooling fluid in the intercooler and the cold side heat reservoir inlet temperature ratio on optimal power and its corresponding intercooling pressure ratio, as well as optimal efficiency and its corresponding intercooling pressure ratio are analyzed by detailed numerical examples. When the heat transfers between the working fluid and the heat reservoirs are executed ideally, the pressure drop losses are small enough to be neglected and the thermal capacity rates of the heat reservoirs are infinite, the results of this paper replicate those obtained in recent literature.

Simulation of multistream plate–fin heat exchangers of an air separation unit

Hot and cold reversible heat exchangers of an air separation unit are simulated. Five fluid streams exchange heat with six fluid streams in parallel and counter flow. The numerical method employed divides the heat exchanger in a number of sections, for which fluid properties, capacity rates and heat transfer coefficients are considered constant. Single and two-phase streams are taken into account. Results obtained from the model are compared with field data.

Power, power density and efficiency optimization for a closed cycle helium turbine nuclear power plant

The performance of a closed cycle helium turbine nuclear power plant for submarine propulsion is optimized in this paper. The power output, power density (ratio of power output to maximum specific volume in the cycle) and thermal efficiency of the cycle are derived. The maximum power, power density and efficiency are obtained by searching for the optimum heat conductance distribution among the hot side heat exchanger (intermediate heat exchanger), cold side heat exchanger (precooler) and recuperator for fixed total heat exchanger inventory with respect to the corresponding optimization objectives. The optimum results are compared with those reported in recent references for the conceptual design of a closed cycle helium turbine nuclear power plant for submarine propulsion. The numerical example shows that the method herein is valid and effective.

Ecological optimization and performance study of irreversible Stirling and Ericsson heat engines

The concept of finite time thermodynamics is used to determine the ecological function of irreversible Stirling and Ericsson heat engine cycles. The ecological function is defined as the power output minus power loss (irreversibility), which is the ambient temperature times, the entropy generation rate. The ecological function is maximized with respect to cycle temperature ratio and the expressions for the corresponding power output and thermal efficiency are derived at the optimal operating conditions. The effect of different operating parameters, the effectiveness on the hot, cold and the regenerative side heat exchangers, the cycle temperature ratio, heat capacitance ratio and the internal irreversibility parameter on the maximum ecological function are studied. It is found that the effect of regenerator effectiveness is more than the hot and cold side heat exchangers and the effect of the effectiveness on cold side heat exchanger is more than the effectiveness on the hot side heat exchanger on the maximum ecological function. It is also found that the effect of internal irreversibility parameter is more than the other parameters not only on the maximum ecological function but also on the corresponding power output and the thermal efficiency.

Intensive energy density thermoelectric energy conversion system by using FGM compliant pads

In order to provide increasingly large amounts of electrical power to space and terrestrial systems with a sufficient reliability at a reasonable cost, thermoelectric energy conversion system by using Functionally Graded Material (FGM) compliant pads has been focused. To achieve high thermal energy density in thermoelectric (TE) power conversion systems, conductively coupling the TE module to the hot and cold heat exchangers is the most effective configuration. This is accomplished by two sets of FGM compliant pads. This design strategy provides (1) a high flux, direct conduction path to heat source and heat sink, (2) the structural flexibility to protect the cell from high stress due to thermal expansion, (3) an extended durability by a simple FGM structure, and (4) manufacturing cost reduction by spark plasma sintering. High thermal energy density of more than twice as much as conventional conduction coupling TE generator is expected. TE energy conversion systems combined with FGM compliant pads for space and terrestrial design options are proposed in this paper.

Heat transfer effect on the performance of MHD power plant

The thermodynamic performance analysis of a magnetohydrodynamic (MHD) power plant is performed in this paper. The analytical expressions for the power output and the efficiency are derived for the MHD plant coupled to variable temperature heat reservoirs at constant gas velocity and constant Mach number conditions. In the analysis, the irreversibilities include the irreversible heat transfer losses in the hot and cold side heat exchangers, the irreversible compression loss in the compressor, the irreversible expansion loss in the MHD generator and the effect of finite thermal capacitance rate heat reservoirs. Detailed numerical examples are given.

Formation of fouling layers on a heat exchanger element exposed to warm, humid and solid loaded air streams

In consideration of the very high energy consumption of drying processes it is astonishing and disadvantageous that the majority of industrial dryers are offered and operated without energy recovery. The avoidance of fouling is here the key issue concerning an efficient heat recovery from dryer exhaust gases. By the present state of knowledge, fouling mechanisms under the specific conditions of dryer exhaust gases––intensified and additionally complicated by the interrelated process of condensation of vapors––are neither clarified nor predictable. The objective of the present investigations is a phenomenological description and quantification of the dynamics of fouling layer formation, with identification of ranges of influential process conditions responsible for the onset of formation of fouling layer, its growth and destruction. To analyze the formation of fouling layers on cold heat exchanger surfaces, a specialized experimental facility, which contains a so-called dryer exhaust gas simulator and a cooled probe tube in a measuring section has been designed and built. In the present, selected results from the first group of experiments carried out with this facility are presented and discussed. Solids in the gas stream have been CaCO 3 with d p =40 μ m, parameters affecting vapor condensation have been varied.

Performance of a thermoacoustic prime mover without a stack

A standing-wave thermoacoustic prime mover which does not contain a stack has been constructed and tested. The device consists of two heat exchangers and a resonator body. The cold heat exchanger may be moved relative to the hot heat exchanger while the resonator is sealed and thermoacoustically generated sound is present. The device has a cross-sectional area of about 500 cm2 and operates near 600 Hz with ambient air as the working gas. It will be explained how a no-stack thermoacoustic device may in principle have a greater efficiency than a stack-based device. In contrast to the well understood stack-based standing wave thermoacoustic engines, it is expected that the efficiency for a no-stack device will be nearly proportional to the acoustic pressure amplitude. Preliminary measurements on the device have achieved acoustic pressure amplitudes of 2.5% of the mean pressure. An interesting result is the presence of spontaneous thermoacoustic oscillations even when the gap between the cold and hot heat exchangers is several times larger than the gas displacement, 2x1. [Work supported by ONR.]

Performance comparison of an endoreversible closed variable temperature heat reservoir Brayton cycle under maximum power density and maximum power conditions

In this paper, the power density, defined as the ratio of power output to maximum specific volume in the cycle, is taken as the objective for performance analysis of an endoreversible closed Brayton cycle coupled to variable temperature heat reservoirs in the viewpoint of finite time thermodynamics or entropy generation minimization. The analytical formulae about the relations between power density and pressure ratio are derived with heat resistance losses in the hot and cold side heat exchangers. The obtained results are compared with those results obtained by using the maximum power criterion. The influences of some design parameters on the maximum power density are provided by numerical examples, and the advantages and disadvantages of maximum power density design are analyzed. The power plant design with maximum power density leads to a higher efficiency and smaller size. When the heat transfer is effected ideally and the thermal capacity rates of the two heat reservoirs are infinite, the results of this paper become those obtained in recent literature.

One-dimensional models for heat and mass transfer in pulse-tube refrigerators

This paper studies systems of partial differential equations modelling pulse-tube refrigerators. Through a regular asymptotic expansion, the pressure is found to be a known function of time. Therefore, models for heat and mass transfer are derived neglecting the equation for conservation of momentum; the cooling mechanism being highlighted in terms of this model. New approximate formulas are derived for the velocity in the tube and the non-linear thermal wave speed in the porous medium (known as the regenerator) on the short (piston oscillation) time-scale. The temperature dependence of the thermal wave speed is proposed as one measure of the efficiency of the transport of heat away from the cold heat exchanger. Progress is limited on the long time-scale, however, the importance of the difference between the gas temperature averaged over an oscillation and heat exchanger temperature is identified.

Thermodynamics of open cycle thermoacoustic engines

Recent advances in thermoacoustics have included the design, fabrication, and testing of an open cycle thermoacoustic refrigerator where the cold heat exchanger can be replaced by a slowly flowing gas which is cooled as it passes through the stack [R. S. Reid and G. W. Swift, J. Acoust. Soc. Am. 108, 2835–2842 (2000)]. In addition to removing the irreversibilities associated with the cold heat exchanger, an important thermodynamic advantage is provided by cooling the working fluid as it flows through the stack, because the removal of heat from the fluid at higher temperatures increases the efficiency of the refrigeration process. This work investigates the possibility of similar efficiency improvements in open cycle thermoacoustic engines and heat pumps, where a steady flow is superimposed on the working fluid. The benefits of adding mean flow to a thermoacoustic engine rely heavily on the chosen method of heat transfer to the hot side of the stack or regenerator. Ideal and nonideal heat transfer from flow-based and electric-based heat exchangers will be analyzed and compared to an open cycle thermoacoustic engine driven by a steady flow of hot gas through its stack or regenerator.

Thermoeconomic considerations in the optimum allocation of heat exchanger inventory for a power plant

Thermoeconomics is defined as the integration of thermodynamics with economics of thermal systems. In this paper, we discuss the thermoeconomics of heat exchanger units in a power plant for cost based optimal design conditions. In this regard, unit cost parameters of hot and cold end heat exchangers in distributing the heat transfer surface area of the power plant are considered for minimum total cost of the heat exchangers. A closed form expression is given in terms of unit costs of the conductances of both heat exchangers, and the results are presented in terms of a unit cost ratio, G, and the hot and cold end heat exchangers costs. The results demonstrate a strong dependence of the total cost function on the absolute temperature ratios as well as the hot to cold end conductance cost ratio. It is also shown that for the case of equal unit costs of the hot and cold end heat exchangers, the total conductance is equally divided between the two heat exchangers.

Characteristics of Coupled Types of Thermoacoustic Sound Wave Generators.

New types of thermoacoustic sound wave generators, which play an important role in the thermoacoustic refrigerator, are developed. The thermoacoustic sound wave generator consists of hot and cold heat exchangers, stack and resonance tube which is made of stainless steel and Plexiglas tubes, 32 mm in inner diameter and 860 mm long. The theoretical sound wave frequency is 100 Hz. First, two sound wave generators are connected with each other either at the open ends or the closed ends. When they are connected at their open ends, their velocity, pressure amplitude and heat transfer are considerably improved compared with those of independently operated ones. Each phase of both velocity and pressure is inverted automatically. However, when they are connected at their closed ends, their velocity, pressure amplitude and heat transfer are almost equal to those of independently operated ones, and each phase is synchronized automatically. Second, a loop-type sound wave generator constructed of four sound wave generators and four bent tubes is developed. Its sufficient performance is also confirmed.