Onset Temperature Difference Research Articles

Investigation on a heat-driven thermoacoustic refrigerator with minimal complexity

Heat-driven thermoacoustic refrigerator (HDTR) emerges as a pivotal solution to global energy challenges and carbon emissions. This study introduces a simplified single-unit HDTR as a new supplement to the heat-driven thermoacoustic refrigeration process. Featuring only one simple, direct-coupled thermoacoustic energy conversion core unit, it is designed to reduce overall system complexity. Initially, the simulation model of the single-unit system is established in Sage and compared with a two-unit HDTR, indicating that reducing the number of thermoacoustic core units decreases system complexity without compromising system performance. The onset characteristics and steady-state operational characteristics of the proposed single-unit HDTR are then experimentally tested, revealing a cooling power of 895 W with a coefficient of performance (COP) of 0.43 when the ambient and cooling temperature is 25 °C and 0 °C, respectively. Notably, a minimal onset temperature difference of 61.6 °C is obtained for the single-unit HDTR. These results showcase the possible application potential for low-grade heat recovery. A comparative analysis against existing loop HDTR systems establishes its superior performance, offering a valuable reference for contemporary HDTR assessments. This study significantly advances HDTR development, emphasizing efficiency and sustainable cooling applications.

First experimental realization of a thermoacoustic-based flue-gas analyzer

Flue-gas analyzers are critical for various applications, including the optimization of combustion efficiency and air quality monitoring. While capable of accurately measuring gas concentrations, current sensors require frequent calibration, suffer short lifespans, and can be susceptible to inference from trace gases. In this report, we provide a first physical implementation of a thermoacoustic-based gas analyzer by investigating the sensitivity of a standing-wave thermoacoustic engine to variations in three different flue-gas components, CO2, O2, and N2. Experimentally measured shifts in both the onset temperature difference and the fundamental resonance frequency with varying gas compositions confirm the feasibility of a thermoacoustic system for flue-gas analysis. The presented data paves the way for further exploration of this innovative approach, offering an efficient and cost-effective acoustic-based alternative to existing flue-gas analysis methodologies.

Influence of resonator length and temperature configuration on the performance of a supercritical CO2 thermoacoustic engine

The peculiar properties of CO2 at supercritical pressures and transcritical temperature conditions are favorable for thermoacoustic devices. The printed circuit heat exchanger has been employed to fabricate a standing-wave thermoacoustic engine that works under these conditions. Experiments performed in this paper are devoted to exploring the influences of resonator length and temperature configuration on its performance. Results demonstrate that the oscillation frequency drops when the resonator is elongated, while the pressure amplitude is strengthened. To generalize the experimental data, the power-law relation for pressure amplitude and the bilinear law of frequency are extended to include resonator length as an independent variable, serving as the basis for tuning performance through resonator length. The best temperature configuration is to put the pseudo-critical temperature at the midpoint of heating and cooling temperatures to induce the pressure oscillation at maximum amplitude. Guided by the above results, experiments are performed to explore the extremum of low onset temperature difference. The current record is finally identified as 8.6 °C, close to the literature record of 7.0 °C. This paper elucidates the effects of resonator length and temperature configuration. Conclusions drawn in this paper can guide the design and operation of thermoacoustic devices working with supercritical CO2.

Prediction of Onset Temperature in Standing Wave Thermoacoustic Engine with Mesh Screen Stack

A thermoacoustic engine is a engine that converts thermal energy into acoustic energy, which can be used to generate electricity or cooling. This engine is attractive because it consists only of a stack, heat exchangers, and a resonator. The stack serves as the primary component for the energy conversion process and consists of porous materials like an array of stainless steel mesh screens. To generate the acoustic energy, a minimum temperature difference is necessary between the two sides of the stack, called the onset temperature difference. However, the calculation for prediction of onset temperature on the stack made of mesh screen has not been addressed. Therefore, the objective of this paper is to propose a method that can be used to estimate the onset temperature difference in standing wave thermoacoustic engine with stacks made of mesh screen arrays. The onset temperature difference is predicted numerically using linear stability theory and matrix transfer methods. Experimental verification is carried out by using standing wave thermoacoustic engine from pervious study. The results showed that the lowest onset temperature difference (TH - TC = 140ºC) is obtained when rh = 0.497 mm. Furthermore, the numerical and experimental onset temperature difference comparisons show a qualitative agreement, allowing the onset temperature prediction method to be used in designing standing wave thermoacoustic engines with stacks made of mesh screens.

Effect of channel size on the performance of a PCHE-based supercritical [formula omitted] thermoacoustic engine

Thermoacoustic engine is a promising energy conversion device. Recently, supercritical CO2 has been successfully employed as the working fluid in a standing-wave thermoacoustic engine operating under transcritical temperature conditions. The core components of the device are integrated as a printed circuit heat exchanger (PCHE), where the CO2 channel size plays a vital role in the performance of the device. In the present work, guided by approaching two to four times thermal penetration depth, a PCHE module with improved channel size is designed, fabricated, and tested. Results suggest improvements are achieved in pressure amplitude Δp (the highest record increases from 0.419 to 0.455 MPa) and onset temperature difference (the lowest record drops from 14.8 to 11.2 °C), which are attributed to the enhanced thermally-driven expansion and contraction during one oscillation period. This paper shows that the average thermal penetration depth is a notable variable generally positively correlated to the applied temperature difference. By studying the transition and saturation in operation mode observed in the experiments, it is revealed that three to four times the average thermal penetration depth is the best interval for the CO2 channel size. Beyond this interval, the power law relation between dimensionless pressure amplitude and the temperature difference holds. Below this interval, the saturation becomes so strong that the growth of pressure amplitude with temperature difference decelerates significantly.

Experimental investigation of a modified parallel stack for wet thermoacoustic engine to improve performance and suppress harmonics

When compared to the ‘classical’ thermoacoustic engines, the major drawbacks of Wet Thermoacoustic Engine (WTE) are their ambiguous mechanism of condensable liquid ingestion, cessation of continuous operation and the domination of higher harmonics. In this study, an approach to address these issues is presented via a novel design of stack. A closely packed mesh screen matrix is inserted between the two plates of ‘parallel stack’, which is intended to contain condensable liquid for WTE. This modified design is proposed to gain from the enhanced heat transfer area and the prevention of higher harmonics production. The effect of the different geometrical features and design parameters of the stack on the engine's performance is investigated experimentally for a standing wave WTE. Onset temperature difference, acoustic power, harmonics ratio and fundamental frequency are measured for the variations in plate spacing, mesh packing density, mesh number, and mesh packing length. An optimum design of the stack within the parametric space of the study is obtained and the underlying physics behind the observed behavior is reported. For the optimum configuration, the generation of higher harmonics is reduced to 58% and the maximum acoustic power is generated at a low onset of 13∘C.

Experimental investigation of a novel standing-wave thermoacoustic engine based on PCHE and supercritical CO2

The pursuit of high amplitude and low onset temperature difference is one important topic in thermoacoustic engines. In this paper, a novel standing-wave thermoacoustic engine is constructed working with supercritical CO2, whose core components are integrated as a printed circuit heat exchanger (PCHE). Our experimental results confirm that at any location on the resonator, the pressure obeys a cosine law with time, and the whole pressure field exhibits a seesaw-like spatiotemporal behavior. Under a fixed charging mass, the amplitude of pressure oscillation increases with temperature difference, obeying a power law relation. The maximum amplitude achieved in this work is 0.419 MPa at a temperature difference of 151.0 °C and a mean pressure of 9.621 MPa. Besides, the frequency with an average of 7.6 Hz shows weak variations. The onset temperature difference decreases as the critical pressure is approached, and the minimum value achieved is 14.8 °C. Moreover, the cooling effect of oscillating CO2 to the high-temperature portion of the stack is revealed, causing the strongly nonlinear temperature profile along the stack. This paper manifests that supercritical CO2 and PCHE are highly prospective to be employed in thermoacoustic devices.

Time-domain acoustic-electrical analogy investigation on a high-power traveling-wave thermoacoustic electric generator

Thermoacoustic electric generator is a novel heat-to-power conversion technology with potential high efficiency and reliability, showing great potential in distributed power generation. In this paper, we conduct time-domain acoustic-electrical analogy investigations on a high-power 4-stage looped traveling-wave thermoacoustic electric generator to shed lights on the system performance. The components of a thermoacoustic system are simplified as an acoustic–electrical analogy model, thus saving computational time. Additionally, the nonlinear effects in the regenerator, heat exchanger and resonance tube are considered in establishing the time-domain equations. Method validation shows a reasonable agreement between experimental and simulated results. Transient evolutions of the oscillating pressure and volume flow rate are firstly given. Investigation is then conducted on onset temperature using different working gases of helium, argon, nitrogen, and neon. The results indicate that when the external resistance reaches 70 Ω, the minimum onset temperature difference of 112 K can be achieved with argon as the working gas. For steady-state characteristics, the influences of electric resistance and heating temperature are explored. It is found that higher heating temperature contributes to larger electric power. A large electric resistance results in a large electric resistance at high heating temperature, while the highest efficiency is achieved with a medium electric resistance. Furthermore, higher heating temperature can lead to more irreversible losses. With helium employed, a maximum electric power of 13.3 kW with thermal-to-electric efficiency of 27.4% is achieved with an electric resistance of 150 Ω and a heating temperature of 950 K, and a highest thermal-to-electric efficiency of 33.5% with electric power of 1.38 kW is obtained when the electric resistance and heating temperature are 90 Ω and 750 K, respectively. This study shows that the proposed method provides a new perspective and an effective method for understanding thermoacoustic electric generator.

Numerical simulation of nonlinear phenomena in a standing-wave thermoacoustic engine with gas-liquid coupling oscillation

• A CFD model of gas-liquid standing-wave thermoacoustic engine is developed and experimentally verified. • Time-domain impedance boundary condition is employed to truncate the modelling of liquid column. • Entire transient evolution of self-excited thermoacoustic oscillation from onset to saturation is captured. • Three different acoustic streamings are observed through inspecting the time-averaged mass flux field. A computational fluid dynamics (CFD) model based on the compressible, nonlinear Navier-Stokes equations is developed to simulate the nonlinear phenomena in a gas-liquid standing-wave thermoacoustic engine, which employs gas as the working fluid and liquid as the phase-matching element. The CFD model is well validated when comparing the simulated onset and steady-state performances with those measured from a real experimental system. Based on the CFD model, the nonlinear phenomena of the engine are analyzed from three aspects. Firstly, the nonlinear dynamic phenomena of self-excited thermoacoustic oscillation from onset to saturation are investigated, focusing on onset temperature difference and steady-state pressure amplitude. Then, the nonlinear acoustic phenomena of the engine operating at steady state are analyzed. The steady-state fluctuating pressure is decomposed into a superimposition of the acoustic modes with different frequencies, which reveals the existence of high-order harmonics. Finally, the nonlinear hydrodynamic phenomena including multi-dimensional flow effect and mass streaming are observed by examining the steady-state flow fields in the engine. The proposed CFD model provides an effective approach to comprehending the mechanism of nonlinear phenomena in gas-liquid coupling thermoacoustic engines.

Developing an augmented onset model for a thermoacoustically-driven, pulse tube cryocooler

This paper aimed to develop an augmented onset model for a looped-branched, thermoacoustically-driven, pulse tube cryocooler based on the transfer matrix model, introducing a reliable, uncomplicated prediction of the onset features compared to a full transient simulation of the thermoacoustic field. Disciplined successive multiplication of the element's matrices extracted from the linear thermoacoustic equations along with the joint, boundary and periodic conditions establish an eigenvalue onset problem. The corresponding problem solutions are in line with the earlier experimental results for a looped-branched thermoacoustic system. Investigating multiple layouts with specific startup characteristics indicated the ability of a four-stage cryocooler driven by two engines at each stage to reach an onset temperature difference of 45 K at liquid nitrogen temperature. Furthermore, the Morris screening method was adopted to rank the uncertain input factors, followed by the Sobol sensitivity analysis to quantify the uncertainty of model outcomes. The sensitivity quantification highlights that the uncertainty in the gas equation-of-state, specific heat capacity, and the cold regenerator mesh number predominantly affect the onset features. Other influential factors include the dimensions of the hot and cold segments of the unit that respectively affect the onset temperature and the frequency.

A closed two-phase thermofluidic oscillator with zeotropic mixtures for low-grade heat recovery

A thermodynamic model is developed for calculating the net specific work, thermal efficiency and relative Carnot efficiency of a closed two-phase thermofluidic oscillator using zeotropic mixtures as working fluids. Nine zeotropic mixtures including R401A, R401B, R401C, R407A, R407B, R407C, R404A, R408A and R410A are screened as candidate working fluids for experiments, which are preselected on the basis of the proposed thermodynamic model. The influences of zeotropic mixture on the onset temperature difference and oscillation frequency are then experimentally investigated. The lowest onset temperature difference of 18.6 °C is achieved with R401C as working fluid, showing good applicability to low-grade heat recovery. R410A produces the highest oscillation frequency of 4.9 Hz. Besides, experiments are also performed to examine how pressure ratio is affected by zeotropic mixture. The highest pressure ratio of 1.067 is obtained with R401C as working fluid, when the hot temperature is 187 °C. The system with R410A has the potential for higher energy density thanks to its higher operating pressure. For the first time, this work verifies the applicability of zeotropic mixture for a two-phase thermofluidic oscillator.

Numerical and experimental study of a two-phase thermofluidic oscillator with regenerator achieving low temperature-differential oscillation

In this work, a two-phase thermofluidic oscillator with regenerator is numerically and experimentally investigated, focusing on the mechanism of regenerator in lowering the onset temperature. A modified acoustic-electric analogy model, considering the thermophysical properties of regenerator, is developed to predict the onset temperature difference and resonant frequency at onset point, which is then verified by experiments. The influences of the material and type of regenerator on the onset temperature difference, resonant frequency and pressure amplitude have been investigated. By inserting the regenerator packed with copper mesh screens, the onset temperature difference is decreased from 35.1 °C to 7.0 °C, which is lowest ever reported in the literatures. Additionally, the pressure amplitude is increased from 4.5 kPa to 10.7 kPa at a hot temperature of 46 °C. The merits of simple construction, free maintenance and low temperature-differential onset enable this updated two-phase thermofluidic oscillator suitable for low-grade heat harvesting.

Study on a heat-driven thermoacoustic refrigerator for low-grade heat recovery

Recovering low-grade heat from renewable energy sources and waste heat is crucial for improving energy utilizing efficiency as well as reducing CO2 emissions. Conventional thermoacoustically-driven refrigerators have a high onset temperature and low cooling efficiency, which significant limit their capacity for low-grade heat utilization. This paper investigates a novel thermoacoustically-driven refrigerator with gas-liquid resonators which enable a lower onset temperature and better cooling performance for harvesting low-grade heat. Theoretical analyses were performed on multi-stage systems to explore the onset characteristics and steady performance. Onset characteristics analysis was conducted by using a transfer matrix method. The effects of mean pressure, liquid volume ratio and the expected liquid mechanical damping coefficient on the onset temperature difference and working frequency were studied for systems with different numbers of stages. A comparison of system onset performance was made with conventional systems containing a gas-only resonator. The research illustrated that for a mean pressure of 1 MPa, the proposed system can significantly reduce the onset temperature difference from 144.1 K to below 35.5 K. In addition, an analysis was then conducted to study the parametric sensitivity of the thermodynamic performance. Calculation results show that the proposed system can achieve a baseline cooling power of 2.7 kW and a thermal-to-cooling efficiency of 0.67 at a heating temperature of 420 K and a cooling temperature of 270 K. This represents significant increases by a factor of 5.6 in cooling power and 1.5 in efficiency from a gas-only to a gas-liquid resonator.

Physicochemical and functional properties of cassava flour grown in different locations in Sabah, Malaysia

The tuber of cassava is used as raw materials in the bakery, food, pharmaceutical and garment industries. The nutritional value of cassava roots is important because they are the main part of the plant consumed in developing countries. However, there is much variation in the nutrient quality of the cassava root depends on the several factors, such as geographic location, variety, age of the plant, and environmental conditions. This study was performed to compare and provide information on physicochemical and functional properties of cassava flour planted in two different districts in Sabah, Malaysia, namely Tawau and Semporna. Proximate analysis showed significant differences (p<0.05) in crude protein (2.07 and 2.69%), crude fat (0.55 and 0.68%) and dietary fibre contents (2.38 and 2.09%). Determinations on physicochemical and functional characteristics of the cassava flour showed significant differences (p<0.05) in bulk density (0.57 and 0.79 g/ cm3 ), pH (6.75 and 6.72), colour and foam capacity (3.66 and 7.33%) while there was no significant difference shown in water and oil absorption capacities as well as emulsion capacity. Cassava planted in Semporna was observed to have high values of all pasting property parameters relative to the one planted in Tawau except for the setback viscosity. Gelatinization properties of flours showed significant differences (p<0.05) in onset (70.59 and 68.99°C) and end temperatures (79.81 and 80.03°C).

Electrical-analogy network model of a modified two-phase thermofluidic oscillator with regenerator for low-grade heat recovery

An electrical-analogy network model including a regenerator model based on wet thermoacoustic principle is proposed to predict the resonant frequency and the onset temperature difference of a modified two-phase thermofluidic oscillator, which is similar to a non-inertive-feedback thermofluidic engine reported in the literature. Besides, a screen-stacked regenerator is introduced to reduce the irreversible loss induced by heat transfer and a gas reservoir is inserted to tune the acoustic field. An electrical capacitance that accounts for the compressibility caused by thermal-relaxation effect and an electrical resistance that accounts for the heat loss due to viscous dissipation are included in the regenerator model, with all other components modeled based on lumped principle. The electrical-analogy network model can well predict the onset temperature difference in both trend and magnitude, with which the deviations from experimental results are only 0.8–5.8% for the present model, while they are 68.5–72.0% for the previous linear temperature profile heat exchanger model. In addition, the calculated and experimental resonant frequencies are also in reasonable agreement, since the present model overestimates the measured resonant frequencies by 1.5–12.5%, while the previous model overestimates by 5.2–16.5%. The effects of the liquid column length inside the load tube, the gas reservoir volume and the working fluid on the resonant frequency and the onset temperature difference are investigated. The results from this work can help to better understand and then optimize a two-phase thermofluidic oscillator.

Development of a Multi-Stage Configuration for Electricity Generation Using Thermo-Acoustic Approach

This work describes the development and assessment of a three-stage thermo-acoustic electricity generator that demonstrate the potential of conversion of waste heat into useful electricity. A three-stage travelling-wave thermo-acoustic system was constructed and tested. The effect of the heat source on the potential of the device to generate electric power is analysed. Theloudspeakerhad used for the acoustic-to-electricity conversion in this research study. The onset temperature difference, the onset time,and the magnitude of the electricity generated has considered as the performance indicators of the device developed. This paper provides details of the experimental investigation that consists of measuring the temperatures, the frequency of the sound-wave, and the acoustic power generated using thermocouples and sound level meter coupled to data acquisition for signal processing and visualization. Electric power supplied to the cartridge heaters at a maximum voltage range of 150 to 190 Volts. The effect of the geometrical configuration of the device concerning acoustic-to-electricity with electricity conversion is analysed. The best geometrical arrangement for generating electricity at the lowest onset temperature difference had identified.The three-stage travelling-wave system generated the highest output voltage of approximately 2.4 V.

Numerical analysis on thermoacoustic prime mover

A two-dimensional numerical model based on compressible SIMPLE algorithm was developed to simulate the evolution process of self-excited oscillation in a thermoacoustic engine at two different heating conditions. Moreover, the performance analysis of the thermoacoustic engine was researched. On one hand, the influence of stack parameters and charge pressure on the onset temperature of self-excited thermoacoustic oscillation was exhibited and analyzed. On the other hand, the impact of different gas types on the onset temperature difference was investigated. The results indicated that the self-excited thermoacoustic oscillation happened with the same performance for both two heating conditions. The minimum onset temperature difference can be achieved when the dimensionless stack length and position are Lr = 0.055–0.06 and Xr = 0.14–0.16, respectively. For a detail, the optimal stack plate thickness is 33% of the stack channel width when the computational width is 3 times as large as the thermal penetration depth of the working gas. Furthermore, the onset temperature difference increased with the increasing of the charge pressure in prime mover. For different working gas, the onset temperature difference was the highest for argon, which was followed by helium and nitrogen. Finally, the performance was improved with the increase of the temperature difference between the two exchangers in the prime mover.

Onset and damping characteristics of a closed two-phase thermoacoustic engine

Onset and damping processes that characterize the transition of a thermoacoustic engine between the stationary and periodic oscillating states have attracted much research effort. In this work, the onset and damping characteristics of a closed two-phase thermoacoustic engine are investigated, where a regenerator is inserted between the cold and hot heat exchangers to reduce the irreversible loss caused by heat transfer. Additionally, a branch resonator, which consists of a load tube and a gas reservoir, is introduced to form the closed system and to adjust the acoustic field. A lumped parameter model is proposed to quantitatively analyze the performance of the thermoacoustic engine. Upon optimization, an onset temperature difference as low as 8.2 °C can be achieved in the experiments with R134a as the working fluid, which is the lowest one ever reported in the literatures. Besides, hysteresis phenomenon is found during the onset and damping processes. The present work aims to provide better understanding of the onset and damping behaviors of a two-phase thermoacoustic engine.

Modelling and analysis of a thermoacoustic-piezoelectric energy harvester

This paper proposes an energy harvester that can convert the acoustic power produced by a standing-wave thermoacoustic engine into electricity using a piezoelectric transducer. The proposed thermoacoustic-piezoelectric system may start producing electricity when sufficient heat input is supplied and large-amplitude thermoacoustic oscillations are generated. The stability of the thermoacoustic-piezoelectric energy harvester is analysed using a transfer matrix method based on linear thermoacoustic theory. Experimental results are presented to validate the predicted onset temperature difference across the stack and oscillation frequency. Subsequently, the acoustic field, viscous and thermal losses and acoustic power generation along the tube at the threshold condition are analysed. The present work also focuses on the effect of the temperature distribution of the system on the onset characteristics and the energy conversion process. In addition, a parametric study is performed to investigate the effect of the main geometrical and electrical parameters on the onset characteristics of the entire system. The analytical and experimental approaches developed in this study are useful for the design and optimization of thermoacoustic-piezoelectric engines for harvesting waste thermal energy in industrial applications.

Performance optimization of the regenerator of a looped thermoacoustic engine powered by low-grade heat

This paper focuses on the influence of regenerator on the performance of a single-stage looped thermoacoustic engine. Numerical simulation has firstly been carried out to investigate how the regenerator efficiency, generated acoustic power, and acoustic field in the regenerator are affected by the cross-sectional area, regenerator length, and mesh number. Based on the numerical optimization, an experimental setup with a normalized cross-sectional area of 14.5 and a normalized regenerator length of 0.0036 has been established, and the influence of the hydraulic radius of regenerator (determined by regenerator mesh number) on the onset process and the stable oscillation of the system has been studied. The results show that the ratio of the hydraulic radius of regenerator over thermal penetration depth should be in the range of 0.2 to 0.3, in order to achieve a low onset temperature difference and a high thermal efficiency. In the experiments, an onset temperature difference of 84°C and a thermal efficiency above 4% at a temperature difference of about 200°C were obtained, showing the potential to recover low-grade heat.