Ambient Heat Exchanger Research Articles

Experimental validation of a heat exchanger model for thermoacoustic applications

Thermoacoustics is a promising technology for energy conversion purposes. Among the bottlenecks limiting a large diffusion of thermoacoustic devices, there are heat exchangers, whose behaviour in oscillatory flows is rather different than those working in stationary flows. Furthermore, the classical linear acoustic theory in the frequency domain cannot predict with high-fidelity the thermo-fluid dynamics of such heat exchangers. In this article, a CFD model based on a standing wave device including a parallel plate heat exchanger is proposed. The setup is inspired by a similar prototype of a thermoacoustic engine in which the performance of the ambient heat exchanger was tested. The results of the CFD model are therefore compared, in terms of the temperature difference between fluid and solid wall in the heat exchanger, average heat flux and Nusselt number, with experimental data showing a satisfying agreement.

A numerical investigation on stack sides thermo-gas dynamic in a large amplitude standing wave thermoacoustic engine

The non-linear unsteady thermal field of a standing wave thermoacoustic engine (SWTE) was investigated using a finite volume computational code in a two-dimensional domain. The transient interaction between temperature and pressure fields and the level of non-linearity especially close to and out of stack plates are studied in 3 cases with different hot heat exchanger (HHX) temperatures of 700,500,450 K and ambient heat exchanger with constant temperature of 300 K. The results show regions with extremum temperatures far different from our understanding of the linear theory of thermoacoustic and the second law of thermodynamics. The gasdynamic field (pressure oscillations) close but out of the stack plates, approached its saturated single harmonic quasi-steady state in a short time (less than 0.3 s), but afterward the thermal field starts and keeps a non-sinusoidal temperature variation. Particle tracking through a Lagrangian analysis revealed that the difference of the thermodynamic cycles for particles that cross a particular region adjacent to the heat exchangers leads to these non-harmonic patterns. Furthermore, in case 1 with HHX temperature and drive ratio of 700 K and 18.2%, respectively, time-averaged temperature extremums with –23 K and +49 K differences (in comparison to AHX and HHX, respectively) were found in the final quasi-steady-state solution. The temperature differences become less considerable when the Drive ratio decreases. For the low temperature close to the AHX, Hybrid Eulerian-Lagrangian analysis, and temperature-entropy diagrams were used to explain this strange physics of generation of extremum temperatures. Thermo-acoustic cycles for ten gas parcels are analyzed to show that parcels in the resonator section visiting shortly the AHX (or HHX) region are responsible for this phenomenon. The thermodynamic cycle for different particles, including interactions of the gas-dynamic field with the thermal field, are analyzed with different tools to justify its strange physics. It is concluded that linear theories do not observe important phenomena in thermoacoustic engines with high-pressure amplitudes. A similar logic holds for the formation of a hot temperature region out of the HHX side of the stack.

Design and Optimization of a Standing-Wave Thermoacoustic Refrigerator Using DELTAEC


 
 
 
 Thermoacoustic refrigeration is one of the best alternatives to conventional refrigeration systems due to the use of inert gases such as helium or air. On the contrary, conventional refrigeration systems use refrigerants (CFC) that are harmful to the environment and humans due to global warming and ozone layer depletion issues. It is possible to classify thermoacoustic technology as a clean, renewable technology with a bright future due to its many advantages. Thermoacoustic refrigerators' main function is to utilize sound waves to create a cooling effect. In the present study, the focus is on the design of a standing-wave thermoacoustic refrigerator driven by an ordinary loudspeaker using the simulation program DELTAEC. Additionally, investigating the influence of significant stack and resonator parameters (inertance and compliance) on cooling power and performance coefficient (COP). Hence, the designed thermoacoustic refrigerator performed well in respect of cooling power and coefficient of performance. It has achieved a cooling power of 312 W with a temperature difference of 25 K between the ambient and cold heat exchangers at a COP of 1.9275.
 
 
 

Design methodology of standing-wave thermoacoustic refrigerator: theoretical analysis

Thermoacoustic refrigeration systems are one of the best alternative solutions for conventional refrigeration systems that are harmful to the environment and humans due to global warming and ozone layer depletion issues. Thermoacoustic technology can be considered a renewable and clean technology with a promising future for its many advantages. A thermoacoustic refrigerator converts acoustic energy to thermal energy (creating a cooling effect). In the present research, the focus is on the design of a standing-wave thermoacoustic refrigerator driven by an ordinary loudspeaker using the numerical simulation program DELTAEC with the concern of building the apparatus at a low cost. In addition, investigating the influence of some crucial parameters on cooling power and thermal/overall performance. Hence, the designed thermoacoustic refrigerator performed well in respect of cooling power and coefficient of performance. It has achieved a cooling power of 134.34 W with a temperature difference between the ambient and cold heat exchangers of 25 K at a COP of 1.956 and the overall efficiency (electrical power converted into cooling power) amounted to 113.43%. The achieved cooling power and COP could be significant when compared to others’ results.

Experimental study on a two-stage large cooling capacity stirling cryocooler working below 30 K

Developing superconducting technology and hydrogen energy system requires large cooling capacity devices working below 30 K. Two-stage Stirling cryocooler has the advantages of large cooling capacity, high efficiency, fast cool-down process, and compact configuration, etc. A two-stage Stirling cryocooler driven by crank-rod mechanism was developed and studied experimentally. The operating characteristics, including the cooling down processes, the pressure characteristics, the heat transfer characteristics of the ambient heat exchanger, and the cooling performances, are analyzed. Under 2.2 MPa charging pressure, a no-load refrigeration temperature of 15.84 K was obtained. With no heat load on the first stage, the cryocooler was capable of providing a cooling power of 138.1 W@30 K with a relative Carnot efficiency of 12.87 %, which is one of the best results of Stirling-type cryocoolers at present. Additionally, a cooling capacity of 47.4 W at 20 K with the relative Carnot efficiency of 6.44 % was achieved. During the cooling down process, an acceleration of cooling down rate of the second stage was observed, leading to a distinct acceleration phase of the declining rate of compression space’s mean pressure. It was found that the influence of the heat load on the first stage on the second stage’s cooling performance was not remarkable, and the cryocooler can provide around 131 W@30 K or 40 W@20 K cooling power on the second stage while the first stage can provide 100 W of cooling power at 75.01 K or 78.42 K, respectively. These experimental results have laid a solid foundation for a deeper understanding of the cryocooler’s operating mechanism and practical application.

Numerical study on a thermoacoustic refrigerator with continuous and staggered arrangements

Thermoacoustic devices require heat exchangers with oscillating flow, but there is currently no viable design approach for them. A heat exchanger with a staggered structure can efficiently improve the velocity disturbance and promote heat transfer in steady flow. The flow and heat transfer characteristics of a standing-wave thermoacoustic refrigerator and an ambient heat exchanger with staggered parallel plates under the oscillating flow condition are investigated in this study, primarily focusing on the geometric influences and differences between staggered and non-staggered (continuous) arrangements. The CFD simulation is a mainstream tool for the numerical simulation of complex thermoacoustic phe?nomena. The flow field around the stack and heat exchanger plate is simulated by introducing the dynamic mesh boundary conditions. Through numerical simulation, the flow field characteristics of non-linear vortices generation around the heat exchanger are presented. By changing the staggered column number in the ambient heat exchanger, it is observed that the larger the column number of staggered parallel plates, the more significant the refrigeration effect through the thermoacoustic effect.

Traveling-wave thermoacoustic refrigerator with curved and tapered surfaces to minimize streaming

The present article describes a traveling-wave thermoacoustic refrigeration system driven by a thermoacoustic heat engine. The coupled engine-refrigerator system consists of a resonator tube, which the thermoacoustic engine and thermoacoustic refrigerator are coaxially located at the extremities of the resonator tube. A thermoacoustic engine (refrigerator) consists of a regenerator, a hot (cold) heat exchanger, two ambient heat exchangers, and a thermal buffer tube. The cross-sectional area of the engine is lower than the resonator cross-sectional area, so a holder is employed to fix the engine inside the resonator tube. The hemispherical end caps, the inner ducts, and the outer ducts have variable cross-sectional areas designed to create a converging-diverging path for the acoustic wave from the outer space to the inner space of the coaxial ducts. Progressive companies on the lookout for a reliable, maintenance-free, environmentally-friendly cooling technology driven by any heat source are the potential customers of this scalable invention.

Acoustic field improvement through adjustable resonator to enhance the performance of thermoacoustic-Stirling engine

In this study, the simulation and experiment of a thermoacoustic Stirling heat engine (TASHE) with a phase-adjuster are demonstrated. The phase-adjuster is used as a tool to tune the acoustic field of the TASHE in order to maintain the high performance of the system. The core components of the TASHE comprising an ambient heat exchanger, regenerator, hot heat exchanger, and feedback pipe, are located in the torus section. Besides, there is a resonator pipe, connecting to the torus, which acts as compliance to maintain the system stability, and the phase-adjuster is installed at the end of the resonator. Firstly, the TASHE is modeled by DeltaEC to search for the optimal configurations of the prototype. Due to the variations of acoustic load or operating conditions from the design criteria, the engine absolutely cannot provide maximum output power. The proposed phase-adjuster could keep its high efficiency by re-matching the frequency. In the experiments, the self-excited and steady-state temperatures of the TASHE are around 755 K and 670 K, respectively. Here, this TASHE can provide an acoustic power of up to 40 W, and an energy conversion efficiency of 12.03%. In comparison, there is a reasonably good agreement between the measured and DeltaEC simulated results. This can reflect on the preciseness of the proposed model. In the case of the TASHE operated under the off-design condition, these scenarios certainly drop its efficiency. Consequently, the role of the phase-adjuster in improving its performance by tuning the acoustic field is presented here.

Numerical study on a three-stage traveling-wave thermoacoustic generator with a simplified structure simultaneously utilizing cold and low-grade thermal energy

As liquefied natural gas (LNG) is becoming a more and more vital role in the global energy market, the enormous amount of cold energy released during its regasification process has attracted worldwide attention. A three-stage traveling-wave thermoacoustic power generator (TWTAG) was proposed utilizing cold energy and low-grade waste heat simultaneously. Most importantly, a simplified structure was applied instead of the traditional structure with thermal buffer tubes (TBTs) and ambient heat exchangers (AHXs). Numerical results show that the system has superior performance together with a much more compact structure. In order to find the best location of the simplified system for acoustic power extraction, three different load-coupling modes were investigated separately. The three-stage TWTAG operating between 110 and 373 K is able to generate 11.4 kW and 10.8 kW electric power under hot-coupled and mid-coupled modes with 4 MPa helium gas as working gas, respectively. It is very enlightening that the cold-coupled mode can generate more than 15 kW acoustic power and shows the best performance. This work shows that thermoacoustic technologies have great potential in LNG cold energy and waste heat recovery.

Compact Diesel Engine Waste-Heat-Driven Ammonia–Water Absorption Heat Pump Modeling and Performance Maximization Strategies

Abstract An investigation of the best ways to achieve optimal performance from a waste-heat-driven ammonia–water absorption heat pump over a wide range of operating conditions is presented. Waste heat from an 8-kWe diesel engine generator is recovered using an exhaust gas heat exchanger and delivered to the desorber by a heat transfer fluid loop. The absorber and condenser are hydronically coupled in parallel to an ambient heat exchanger for heat rejection. The evaporator provides chilled water for space-conditioning with a baseline cooling capacity of 2 kW. All heat and mass exchangers employ novel microscale geometries. A detailed thermodynamics model is developed to simulate performance and develop strategies to achieve the best performance in both cooling and heating modes over a range of operating conditions. These parametric studies show that improved coefficients of performance can be achieved by adjusting the coupling fluid temperatures in the evaporator and the condenser/absorber as the ambient temperature varies. With the varying return temperatures, the system is able to provide the 2-kW design cooling capacity for a wide range of ambient temperatures.

Numerical study on a two-stage large cooling capacity stirling cryocooler working at 20 K

Large cooling capacity regenerative cryocoolers working at 20 K is indispensable in developing superconducting technology and hydrogen energy systems. Two-stage Stirling cryocooler is promising for its advantages such as large cooling capacity, high efficiency, fast cool-down process, and compact configuration. A two-stage large cooling capacity Stirling cryocooler is designed and numerically studied. The influence of the main parameters of the regenerators and working conditions on the cooling performance is discussed. Acoustic field distribution in the cryocooler is revealed for a deep understanding of the internal working mechanism. The performance of the cryocooler, including the available energy losses and cooling capacity, is systematically analyzed. it is found that the phase of the pressure wave always leads the volumetric flow rate in the two regenerators of the cryocooler, which is quite different from the general view. The ambient heat exchanger causes the most substantial part of available energy losses. Under the mean pressure of 3 MPa, a no-load refrigeration temperature of 11.81 K is achieved while the temperature of the first stage is 46.02 K. With no heat load on the first stage, the cryocooler is capable of providing 147 W cooling power at 20 K with a relative Carnot efficiency of 27.2%. When the two stages work at 77 K and 20 K respectively, the cryocooler can simultaneously provide 274.8 W cooling power at the first stage and 130.2 W cooling power at the second stage, with an exergy efficiency of 0.346.

Miniaturized Traveling-Wave Thermoacoustic Refrigerator Driven by Loudspeaker: Numerical Design

Ordinary refrigeration systems such as vapor-compression refrigerators are the commonly used devices in industry, mostly for their high efficiencies. However, they make a significant contribution to the depletion of Ozone and global warming due to their operational refrigerants. Hence, thermoacoustic refrigeration can be a great alternative candidate which uses inert gases such as air, helium and nitrogen as the primary refrigerant. Thermoacoustic refrigerators convert the acoustic power (sound waves) into a thermal effect (cooling power). Thermoacoustics can be counted as a new technology that has a strong potential toward the development of the thermal applications. This study aims to design and fabricate miniaturized traveling wave thermoacoustic refrigerator which can be driven by an ordinary loudspeaker. The optimized numerical design of the refrigerator shows an overall efficiency (cooling power over input electricity) of nearly 66% at a temperature difference of 25[Formula: see text]K (between cold and ambient heat exchangers). The maximum estimated cooling power is 65[Formula: see text]W at coefficient of performance (COP) of 2.65.

The fuel cell and atmospheric water generator hybrid system for supplying grid-independent power and freshwater

Despite the success of atmospheric water generators for providing drinking water to remote regions, this technology has a high specific energy consumption. This paper proposes to reuse the electrochemical water of the fuel cell for the vapor compression cycle based atmospheric water generator (VCC-AWG); After passing through an ambient heat exchanger to remove the electrochemical waste heat, the fuel cell flue gas that enters the VCC-AWG is at a higher relative humidity than natural atmospheric air, thus the freshwater yield per energy input can be significantly increased. Hence, the FC-VCC-AWG hybrid system is proposed to be a power and freshwater supply of a grid-independent home. The fuel cell model, the vapor compression cycle’s thermodynamic model, and humid air physics are coupled to analyze the overall system in terms of the fuel cell’s working condition, incoming airflow rate, compressor power consumption, and the ambient relative humidity. When RH = 0.75, adding a 2 kW fuel cell generated up to 3 kg/hr of freshwater, which is 50% higher than excluding the FC. The specific energy consumption can be 200 Wh/kg, so the VCC-AWG can be integrated with small sacrifices to the FC power output.

Testing of a three-stage looped-tube thermoacoustic sound generator

Thermoacoustic sound generators are devices that produce sound waves by using thermal energy. The output acoustic oscillations can be used to drive a linear alternator to generate electricity. The devices are environmentally friendly because they do not produce any exhaust gases and they can utilize waste heat to generate the sound. This paper presents an experimental testing of a three-stage thermoacoustic sound generator of a looped-tube type. Each thermoacoustic core was composed of a regenerator, a hot heat exchanger and an ambient heat exchanger. The regenerators were made of a tight stack of 30-mesh stainless-steel screens, with a length of 35 mm. The looped-tube was made of stainless-steel pipes with inner diameter of 68 mm and total length of 4.3 m. Atmospheric air was used as working gas within the looped-tube. Three electric heaters, each with input power of around 394 W, were used as the heat sources and installed at the three hot heat exchangers. It was found that the device started to generate sound when the average temperature difference between the hot and ambient sides of the regenerators reached 255 °C. After 45 minutes operation, the average temperature difference was 538 °C, producing sound waves with a maximum pressure amplitude of 2.6 kPa and acoustic power of around 4.5 W. Additionally, it was observed a sound frequency jump during operation, that was 83 Hz until the temperature difference reached 526 °C and jumped to 339 Hz after that. During transition, both frequencies existed with relatively lower pressure amplitudes.

Parametric study of a free-piston Stirling cryocooler capable of providing 350 W cooling power at 80 K

High cooling capacity free-piston Stirling cryocooler (FPSC) has been considered a promising candidate for high temperature superconducting and gas liquefaction applications, which require a cost effective and highly reliable cryocooler capable of providing large cooling power (several hundred watts to several kW) at liquid nitrogen temperature. This paper introduced an advance in a FPSC capable of providing 350 W cooling power at liquid nitrogen temperature. To reveal the relationship between operating parameters and cooling performance, the integrated impact of mean pressure and working frequency on acoustic impedance, acoustic field as well as exergy loss of cooler is studied numerically. The numerical results indicate that for the current design, by tuning the mean pressure and working frequency, an optimal operating scenario of the refurbished FPSC could be obtained, which leads to a higher thermal efficiency. Based on the optimization, a series of experiments were carried out on the improved experimental setup. A cooling power of 350 W at 80 K was obtained with an input electric power of 3.57 kW and the corresponding overall relative Carnot efficiency reached up to 26.8%. Compared with previous reports, the performance has been greatly improved by 25.2%. In addition, a special attention is paid to the heat transfer at the ambient heat exchanger due to the large detected local solid-gas temperature difference in the previous work. Comparisons of two different types of ambient heat exchanger in calculations and experiments indicate that an even better performance could further be expected by employing a shell-tube type heat exchanger with 1 mm inner diameter and a porosity around 0.16.

A two-stage Stirling cryocooler capable of providing more than 100 W cooling power at 30 K

Regenerative cryocoolers with large cooling capacity at 30 K have a promising prospect in the fields of superconducting technology and gas liquefaction. Compared with other types of cryocoolers, Stirling cryocoolers have the advantages of high cooling capacity, high efficiency, fast cool-down process and compact configuration. A two-stage Stirling cryocooler with large cooling capacity driven by the crank-rod mechanism is developed based on the Sage simulation results, and the preliminary experimental results are presented. Firstly, the pressure characteristics under different working conditions are investigated. The amplitude of the pressure and the pressure ratio at the compression chamber increases as the charging pressure increases. Increasing the refrigeration temperature of the second stage results in the decrease of the pressure amplitude, and the decreasing rate increases as the heat load on the first stage increases. Then, the performance of the ambient heat exchanger is studied. As the charging pressure increases from 2.2 to 2.6 MPa, the heat rejected from the ambient heat exchanger increases from 7.57 to 9.19 kW when there are no heat loads at the two stages of the cryocooler. The heat rejected from the ambient heat exchanger increases by 400 W with every increase of 0.1 MPa of charging pressure. With the increase of the refrigeration temperature of the second stage, the heat rejected from the ambient heat exchanger decreases, and the decreasing rate increases as the heat load on the first stage increases. Furthermore, the influence of the variation of the charging pressure and heat loads at the two stages on the cooling performance is introduced. The temperature of the second-stage cold end heat exchanger drops to 30 K at a nearly constant cooling rate after the startup of the cryocooler. As the charging pressure increases from 2.2 to 2.6 MPa, the cooling rate increases from 16.59 to 19.46 K/min, and the cooling rate increases by about 0.72 K/min for every increase in pressure of 0.1 MPa. Under the charging pressure of 2.6 MPa, the second stage of the cryocooler reaches a no-load refrigeration temperature of 19.83 K, while the temperature of the first stage is 71.2 K. The no-load refrigeration temperature has a weak relationship with the charging pressure. As the charging pressure increases, the increasing rate of cooling capacity and COP with the refrigeration temperature increases. The cryocooler is capable of providing 110 W cooling power at 30 K with a relative Carnot efficiency of 10.96%. This is the best result ever reported in China. What is more, it is found that the cooling power at the two stages has little influence on the cooling performance of each other, which means the cooling power from the first stage can be applied in the pre-cooling process with little influence on the second stage. These results lay a solid foundation for future applications and performance improvement of the cryocooler.

Experimental and numerical investigation of natural convection flows in two horizontal thermoacoustic cavities

This study aims at exploring the potential generation of natural convective flows in thermoacoustic devices. To this effect, two thermoacoustic setups are considered without acoustic oscillations, each setup thus constituting a cavity filled with air and comprising a porous medium heated on one of its ends. In the first configuration, the porous medium is a ceramic stack. In the second configuration, an ambient heat exchanger using circulating ambient water is placed on the other end of the porous medium (which is a regenerator). The wall temperature is measured in several points using thermocouples and natural convection flows developing in these cavities are measured in a vertical plane using Particle Image Velocimetry. Moreover numerical calculations are performed based on simplified 3D configurations, using immersed solid bodies at prescribed temperature to represent the heating or cooling elements and describing the porous media as homogeneous and either isotropic or anisotropic. Navier-Stokes equations under Boussinesq assumption are solved in the entire domain with the addition of a Darcy correction term in the porous media. Excellent qualitative and quantitative agreement on velocity fields is obtained between experimental and numerical simulation results for both configurations, which validates the model description. It is shown that the observed natural convection flows are fully three-dimensional and of significant magnitude so that they could potentially interfere with thermoacoustic effects.

Convection Heat Transfer Coefficients in Thermoacoustic Heat Exchangers: An Experimental Investigation

This paper investigates the thermal performance of thermoacoustic heat exchangers subjected to acoustically oscillating flows. The analysis is carried out by experimental measurements of the heat fluxes sustained by the ambient heat exchanger of a prime mover of the standing wave type. A home-made parallel-plate heat exchanger is considered for the study. The gas-side convection heat transfer coefficients expressed as Nusselt numbers are determined over a wide range of velocity amplitudes of the oscillating flow. The experimental results are then compared to the predictions of a number of theoretical models currently applied in thermoacoustics such as the time-average steady-flow equivalent (TASFE) model, the root mean square Reynolds number (RMS-Re) model, and the boundary layer conduction model. The comparison suggests that the boundary layer model performs better than the rms-Re and TASFE models in predicting the heat transfer coefficients in oscillating flows. The relative difference between the model predictions and the experimental data amounts to 19%. A new correlation law, based on regression of the experimental data, is also proposed.

Substantiating the Diameter of the Liner Pipe of the Earth Heat Exchanger and Providing the Air Supply to It

The purpose of the research done was to increase the efficiency of geothermal ventilation that allows us to use the thermal energy of surface layers of the earth for the cooling and heating of the inflow air by substantiating the diameter of the liner pipe of the earth heat exchanger with the volumetric air supply to it. Mathematical simulation was done based on the equations of hydrodynamics, heat exchange and heat conductivity carrying out the computation experiment based on the method of finite volumes. A preliminary developed mathematical model of the heat exchange process between the air moving in the vertical heat exchanger and the soil in-situ was used. This model interconnects the energy indicators of the earth heat exchanger with its parameters including natural climatic conditions. Temperature fields of the cooled air and the soil in-situ were determined and as a consequence, the effective heat power of the earth heat exchanger, the volumetric supply and the ambient air temperature and heat exchanger operation time were also determined for different diameters of the liner pipe. An efficient heat power of the earth heat exchanger is gradually decreased with time. An abrupt drop in power corresponds to an ample air supply to the heat exchanger and it can be explained by a faster exhaustion of the power potential of soil and high energy inputs for the pumping of air through it. An optimization criterion was the relation of the effective thermal energy produced during the earth heat exchanger operation to the diameter of its liner pipe that was named as the power loading of the earth heat exchanger. Based on the maximization of optimization criterion we substantiated the diameter of liner pipe and that of the earth heat exchanger and the volumetric air supply to it.

Experimental study on the effect of hydraulic radius of stack on the performance of thermoacoustic prime mover with open resonator

<p class="Abstract">Thermoacoustic prime mover is a device which converts thermal energy into acoustic work. The device mainly consists of a resonator pipe, a stack (a porous medium), and two heat exchangers. Four stacks were made of a pile of stainless wire-mesh screens with various mesh numbers of #12, #14, #16, and #18, providing the effective hydraulic radius of 0.32 mm, 0.28 mm, 0.25 mm, 0.23 mm, respectively. Each stack has 4 cm length. By using the wire-mesh screens, it is easier to vary the hydraulic radius of stack than any other material. The stack is sandwiched between a hot heat exchanger and an ambient heat exchanger. The resonator is made of a stainless-steel pipe with one end open and the other closed. The stack and heat exchangers are placed inside a resonator pipe near the closed end. The thermal energy were provided by using an electric heater which is installed at the hot heat exchanger. Ambient water was flowing through the ambient heat exchanger. Sound is produced by the stack when the temperature difference between the both ends of stack reachs an onset point. This experiments were performed by recording the temperatures at both ends of stack and the pressure amplitudes of the sound at several points along the resonator for various hydraulic radius of stack. It is found that the effective hydraulic radius of 0.28 mm gives the lowest onset temperature, the shortest time to reach onset condition, and the highest pressure amplitude which are 314°C, 299 s, and 2.89 kPa, respectively. The frequency of sound is not altered by the change of hydraulic radius of stack.</p>