Direct Contact Heat Exchanger Research Articles

Discussion: “A Multi-Objective Comprehensive Evaluation of Heat Transfer Performance of a Direct-Contact Heat Exchanger” [J. Thermal Science and Engineering Applications, 2023, 15 (1), Article No. (011014)

Abstract The units of entransy dissipation (G) in Nomenclature are wrong. Also, an opinion on the concept of entransy is presented.

Experimental investigation of limit parameters of direct contact condensation in the heat exchanger for waste heat recovery

In the present study, a suitable composition of parameters has been obtained to provide an efficient process of cooling flue gas with complete condensation of water vapour from air-water vapour mixture on a water film in co-current upward flow in the tube of the direct contact heat and mass exchanger. The results showed that the value of the irrigation density depends on the velocity of the air-water vapour mixture and the initial vapour content and should be calculated from an empirical equation. The active pipe height depends on the velocity of the air-water vapour mixture and the initial vapour content and should also be calculated from an empirical equation. For example, if the initial vapour content of the air-water vapour mixture is 11%, the velocity of the mixture is 20.8 m/s the height of the channel should not exceed 0.460 m. The value of the water heating limit temperature increases from 46◦C to 62◦C with a change in the initial vapour content from 11% to 30%. The present experimental results could be helpful in the design of direct contact heat and mass exchangers for waste heat recovery.

Melting and Solidification Characteristic of Mixture of Two Types of Latent Heat Storage Material in Direct Contact Heat Exchanger

ABSTRACT The aim of this study is to develop a latent heat storage system designed to utilize waste heat from industrial processes in the mid-temperature range of 100–200°C. Mannitol and a mixture of mannitol and erythritol, both classified as sugar alcohols, were utilized as phase change materials in this study. The present study investigated the melting and solidification characteristics of the mixture of C M = 70mass% (with a composition of 70% mannitol and 30% erythritol by mass) for latent heat storage at approximately 150°C. Additionally, the study aims to experimentally establish the thermal performance of the mixture of C M = 70mass% in a direct contact heat exchanger during melting and solidification. Comparative analysis with mannitol was conducted to determine the melting and solidification behavior of the mixture of C M = 70mass%. Moreover, the heat transfer characteristics, including heat storage and heat release speed, were investigated. Finally, the heat transfer mechanism of each phase change material (PCM) was considered through the overall heat transfer rate. keyword : Phase change material(PCM), Direct contact heat exchanger, Mixture of C M =70mass%, melting & soidification behaviors, Overall coefficient of heat transfer.

Comparing temperature fluctuation attenuation capabilities and feasible parameter regions of indirect- and direct-contact heat exchangers for an ultra-high precision constant-temperature chilled water system: An experimental and Modelica-based study

Constant temperature and humidity rooms have widespread applications in high-end manufacturing and scientific research. Conventional wisdom primarily focuses on controlling the air temperature at the terminals, i.e., the supplied outlets. For achieving ultra-high precision (e.g., ±0.1 °C), it becomes essential to pre-control the temperature of the chilled water of the air-conditioning system. Nevertheless, there is still limited understanding of this matter. This paper investigated two common temperature fluctuation attenuators for the chilled water system: indirect- and direct-contact heat exchangers, e.g., a plate heat exchanger (PHE) and a water mixing pump (WMP), respectively, as follows. First, Modelica models of the two systems were established and validated by an experimental set-up capable of switching between the PHE and WMP loops. Second, an orthogonal analysis was conducted to evaluate the attenuation effects of four key factors. Results indicated that both attenuators can provide the required constant temperature precision with nuanced differences. The MWP system demonstrated faster settling time, primarily affected by valve adjustment accuracy. In contrast, PHE exhibited superior fluctuation control, with valve adjustment accuracy and input chilled water temperature fluctuation amplitude as critical factors. Finally, feasible parameter regions for the two types of attenuators under ultra-high precision targets were delineated and analyzed.

Increasing the efficiency of heating systems through the use of direct contact heat exchange

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Experimental Research on Effects of Combustion Air Humidification on Energy and Environment Performance of a Gas Boiler

Abstract To increase the waste heat recovery (WHR) efficiency of gas boiler and decrease NOx emissions, a flue gas total heat recovery (FGTHR) system integrating direct contact heat exchanger (DCHE) and combustion air humidification (CAH) is put forward. The experimental bench and technical and economic analysis models are set up to simulate and evaluate the WHR performance and NOx emissions in various operation situations. The results show that when the air humidity ratio elevates from 3 g/kgdry air to 60 g/kgdry air, the dew point temperature increases by 7.9 °C. When the flue gas temperature approaches the dew point temperature, the rate of improvement of the FGTHR system's total heat efficiency notably rises. With spray water (SW) flowrate and temperature of 0.075 kg/s and 45 °C, the WHR efficiency relatively increases by up to 8.4%. The maximum sensible and latent heat can be recovered by 4468 w and 3774 w, respectively. The flue gas temperature can be reduced to 46.55 °C, and the average NOx concentration is 39.6 mg/m3. Compared with the non-humidified condition, the NOx and CO2 emissions relative reduction of the FGTHR system are 61.2% and 8.7%. The payback period of FGTHR system is 2 years. Through simulation, it can be concluded that the decrease in exhaust flue gas temperature and velocity, as well as the increase in exhaust flue gas humidity, has a negative impact on the diffusion of NOx in the atmosphere.

Swept open moving particle reactor including heat recovery for solar thermochemical fuel production

Green hydrogen can be produced by thermochemical redox cycles utilizing concentrated solar energy. A high solar-to-fuel efficiency, as well as a robust system design, is crucial. As lacking heat recovery and ineffective use of the redox material significantly reduce the efficiency, the authors developed a particle based concept incorporating these requirements. A central component is a solid–fluid direct contact heat exchanger in which the sweep gas reduces the countercurrent moving particles. Besides that, a key feature is the continuous production of hydrogen due to a locally separated reduction and oxidation step. The amount of technical challenging reactor parts is reduced, since the concept does not require a window and high temperature vacuum. A steady state modeling approach leads to an evaluation of the whole concept. The reduction reaction, a simplified oxidation process and several parasitic losses are included. By this means, the energy share and the solar-to-fuel efficiency is calculated. Depending on the heat exchanger effectiveness, the results indicate solar-to-fuel efficiencies in the range of 4.2 to 18.7% for the non-optimized base case design.

Quantification and correlation analysis of bubble characteristics and heat transfer performance in direct contact heat exchanger

In this work, a combination of morphological processing and watershed algorithm is proposed to measure bubble size, based on which an improved frame difference method is proposed to obtain bubble velocities with an F1 score of 0.96. Empirical equations for the boiling properties of flow in immiscible fluids and their influencing factors are developed. Correlation equations for the Nusselt (Nu) number and Colburn (j) factor are developed, with mean deviations of 5.42% and 5.47%, respectively. The results show that the increase in bubble rise rate and the trend of bubble growth become slower with the increase in the flow rate of the dispersed phase. In contrast, the temperature of the continuous phase has a negative correlation on the bubble growth and almost no effect on the bubble velocity. In addition, the heat transfer coefficient, which increases with the increase of the flow rate of the dispersed phase, has a negative correlation with the initial temperature difference. The main objective of this study is to provide a new method for measuring bubble parameters during direct contact boiling heat transfer and to provide new ideas for studying direct contact boiling heat transfer.

Void fraction measurement in direct contact heat exchange process using electromagnetic tomography

In cooling towers and condensers, accurate measurement of the void fraction in gas-liquid two-phase flow can help optimize heat transfer efficiency. By monitoring the void fraction, operational parameters can be adjusted to maximize heat exchange effectiveness. As such, the development of a technique for measuring void fraction in high-temperature and opaque containers is necessary. This paper presents a method for measuring the void fraction of the section during direct contact heat transfer using electromagnetic tomography. In order to enhance the magnetic field, a certain amount of nanofluid is introduced into the direct contact evaporator. With the aid of a self-developed EMT system, the gas and liquid phases are differentiated, ultimately yielding the section void fraction value. The final results are qualitatively consistent with three classical models for void fraction calculation. This is the first report on measuring cross-sectional void fraction in direct contact heat transfer processes using the EMT system. The findings demonstrate the feasibility of the measurement method based on EMT and provide a reference for research on high-precision and automated measurement techniques of void fraction in gas-liquid two-phase flow.

Accurate characterization of bubble mixing uniformity in a circular region using computational geometric theory

The present study proposes a novel method based on the geometric theory for measuring the distribution of bubble swarms in the circular region of a direct-contact heat exchanger. It was determined that the mixing is uniform when the average distance between the bubble swarms in the unit circular region is approximately 0.9054, which is the standard reference value. The effect of sample size (i.e., the number of bubbles) on mixing uniformity was investigated to determine the optimal sample size. It was verified that the metric's accuracy and stability were higher with a sample size of 155. Accordingly, it was proposed to increase the sample size by filling irregular bubbles using a segmentation method, which enabled a further accurate assessment of the mixing uniformity. The mixing uniformity of bubble swarms in the circular region and its maximum internal connection with the square region was accurately quantified. It was revealed that the relative average error increased by approximately 3.47% due to information loss. The proposed method was demonstrated to be rotationally invariant. The present study provided novel insights into evaluating mixing uniformity, which would guide enhanced heat transfer and the effective evaluation of the spatiotemporal characteristics of transient mixing in circular regions or the cross-sections of chemical transport pipelines.

Numerical Simulation on Enhanced Heat Transfer and Flow Characteristics in Direct-Contact Heat Exchanger: A Novel Kenics Static Mixer

In this paper, a novel Kenics static mixer applied in a direct-contact heat exchanger (DCHE) was proposed in order to enhance the heat transfer efficiency. The computational fluid dynamics (CFD) simulation method was adopted to investigate the effect of the continuous phase and dispersed phase on direct-contact heat transfer (DCHT) in the Kenics static mixer. The perforation index (PI) was introduced to depict the performance of the Kenics static mixer, and the effect of different numbers of openings at three initial heat transfer temperature differences (at the inlet of the continuous and dispersed phases) was studied. The simulation results showed that under the same working conditions, a larger initial temperature difference would result in greater differences in the fluid uniformity index (UI), turbulence intensity, and time evolution of a gaseous working substance. The increase of hole numbers and decrease of hole diameters can optimize the DCHT. Under three initial temperature differences, the heat transfer efficiency for the 6-hole and 9-hole Kenics DCHEs was about 20% higher than the Kenics DCHE without holes.

Packed bed thermal energy storage system using form-stable high-density polyethylene

Energy storage is a pressing need throughout a range of applications, and storage of thermal energy is an increasingly important element in energy management. This study describes the implementation and performance characterization of a new latent heat thermal energy storage system applicable to medium temperature processes requiring heat below 125 °C. The system utilizes a packed bed of form-stable polymer latent heat storage media. The media consists of a high-density polyethylene composite reinforced with glass fiber and a thin surface coating of epoxy resin. Stabilization with traditional composite synthesis approaches provides potential for low cost and high performance. The media is stable over repeated melting/solidification cycles and shows excellent thermal capacity, with more than 160 kJ/kg attributable to latent heat. We characterize the performance of direct contact heat exchange between the storage media and heat transfer fluids. The study includes effects of the mass flux of the heat transfer fluid, fluid inlet temperature during charging and discharging, and initial temperature of the porous bed. The system can be charged and discharged at relatively high rates, e.g. > 100 W/kg, and provides excellent energy density with high exergetic efficiency averaging approximately 79% (i.e. low temperature differences between charge and discharge). The approach presented offers opportunities to enhance the use of thermal storage in medium temperature applications (e.g. a charge/discharge operational range between 120 °C – 140 °C). • Form-stable, fiber reinforced, polymer thermal storage media was synthesized. • Packed bed thermal energy storage performance was characterized. • Media shows excellent latent heat capacity exceeding 160 kJ/kg. • System can be charged and discharged at high rates, e.g. > 100 W/kg. • System can operate with low temperature differences between charge and discharge.

Investigation of Latent Heat Thermal Energy Storage Type Direct-Contact Heat Exchanger

Investigation of Latent Heat Thermal Energy Storage Type Direct-Contact Heat Exchanger

Direct Contact Condensers: A Comprehensive Review of Experimental and Numerical Investigations on Direct-Contact Condensation

Direct contact heat exchangers can be smaller, cheaper, and have simpler construction than the surface, shell, or tube heat exchangers of the same capacity and can operate in evaporation or condensation modes. For these reasons, they have many practical applications, such as water desalination, heat exchangers in power plants, or chemical engineering devices. This paper presents a comprehensive review of experimental and numerical activities focused on the research about direct condensation processes and testing direct contact condensers on the laboratory scale. Computational Fluid Dynamics (CFD) methods and CFD solvers are the most popular tools in the numerical analysis of direct contact condensers because of the phenomenon’s complexity as multiphase turbulent flow with heat transfer and phase change. The presented and developed numerical models must be carefully calibrated and physically validated by experimental results. Results of the experimental campaign in the laboratory scale with the test rig and properly designed measuring apparatus can give detailed qualitative and quantitative results about direct contact condensation processes. In this case, the combination of these two approaches, numerical and experimental investigation, is the comprehensive method to deeply understand the direct contact condensation process.

Enhanced of fluids mixing and heat transfer in a novel SSK static mixer Organic Rankine Cycle direct contact heat exchanger

In this study, new gas-liquid-liquid three phase direct contact heat exchangers based on SSK mixing elements and annular ribs are proposed to improve the thermal efficiency of the Organic Rankine Cycle. Three types of heat exchangers, including hollow tube heat exchanger, SSK heat exchanger and SSK annular rib combined heat exchanger, are established to compare their heat transfer performance. The heat transfer performance, turbulence intensity, turbulence kinetic energy and pressure drop of three heat exchangers are numerically investigated. The results show that the volumetric heat transfer coefficient, outlet temperature of gaseous working medium, turbulence intensity and turbulent kinetic energy of the combined heat exchanger are significantly higher than those of the SSK heat exchanger and the hollow tube heat exchanger. In addition, the pressure drops of the SSK heat exchanger and the combined heat exchanger are basically the same, which are slightly larger than the pressure drop of the empty tube heat exchanger. In summary, the combined heat exchanger has improved heat transfer performance without causing excessive pressure drop.

Study on the effect of air–water flow rate ratio of heat source tower on the soil heat storage

As direct contact heat exchanger, heat source tower (HST) can absorb the total heat of air by water, and then inject the heat into soil through ground heat exchanger (GHE) for soil heat storage in non-heating season. To achieve an optimal soil heat storage dynamic performance, the air–water flow rate ratio (AWFRR) of the HST and the operation strategy of the soil heat storage system should be optimized. Based on the proposed soil heat storage system and self-built module of the HST, the effects of AWFRR of the HST on soil heat storage characteristics were simulated and discussed in different operation modes with the TRNSYS software. In mode A, the values of AWFRR are the same during the whole non-heating season. While in mode B, the values of AWFRR are different during the transition season and cooling season. The results show that the soil heat storage capacity increases obviously with the increasing of AWFRR under specific water mass flow rate. About 64 % of the total soil heat storage capacity in the non-heating season comes from the soil heat storage capacity by HST in the cooling season. The coefficient of performance (COP) of the soil heat storage system first increases and then decreases with the increasing of AWFRR, and there is an optimal AWFRR under specific water mass flow rate to obtain the highest COP. The amounts of soil heat storage capacity, energy consumption and COP of mode A and mode B are similar, and mode A can be assumed to be the more suitable operating mode for its simple operation in engineering application. After optimization of mode A, not only the operating time of the system is reduced from 4440 h to 3264 h, but also more soil heat injection, less energy consumption and higher COP are achieved. To meet the needed soil heat storage of the proposed building in Changchun, the AWFRR and water mass flow rate in mode A should be 0.7 and 7.7 kg/s respectively.

A Multi-Objective Comprehensive Evaluation of Heat Transfer Performance of a Direct-Contact Heat Exchanger

Abstract Direct-contact heat exchangers have great potential for waste-heat utilization. This study established a multi-objective evaluation model of the volumetric heat-transfer coefficient, entransy dissipation, and uniformity of the temperature difference field of the direct-contact heat exchanger. The optimal operating parameters for the heat exchanger were obtained by experimentally investigating the relationship between the operating parameters and the volumetric heat-transfer coefficient, entransy dissipation, and uniformity of the temperature difference field. The results indicated that the smaller the entransy, the greater the effectiveness of the heat exchanger in the direct-contact heat exchanger. The results also indicated that the volumetric heat-transfer coefficient increased with the increase in the flowrate ratio and initial temperature difference. The study found that entransy demonstrated the smallest dissipation and the largest effectiveness of the heat exchanger when the initial temperature difference was 120 °C and the flow ratio was 6:1. This study has guiding significance for the optimal design of direct-contact heat exchangers.

Visualization of direct contact heat transfer process driven by continuous low-temperature heat source and its performance characterization

ABSTRACT In this study, the uniformity factor of the temperature difference field was introduced into the direct contact heat exchanger by establishing a trace element layer model for the heat exchanger. Furthermore, the uniformities of the temperature and temperature difference field of the direct contact heat exchanger operated using a continuous heat source were analyzed. The heat exchange height was proposed,and its effects on the volumetric heat transfer coefficient and heat-exchanger efficiency were analyzed. The experimental results were consistent with the uniformity principle of the temperature difference field.

Logistic fitting model application for evaluation of gas-liquid two-phase mixing effects

Abstract The evaluation of mixing effects is crucial in industrial production such as chemical and metallurgical industries. An improved Betti number method is proposed by using a gas-liquid top-blow mixing experiment and a direct contact heat exchange experiment. This method is adapted to the comprehensive evaluation of flow mixing and heat transfer performance under conditions of dispersed particles or bubbles where the target is identifiable. The comparison results and reveals that our method can not only portray the mixing effect by the critical point of the area integration curve of the Betti number time series but also the parameters (i.e., slope and intercept) obtained after the area integration of the Betti number curve by the logistic regression model can effectively characterize the mixing time and heat exchange performance. The intercept variation of the area integral of the Betti number curve was found to have a high correlation with the correlation coefficients of mixing time and heat transfer coefficient. Meanwhile, a new index for evaluating the mixing performance was obtained. The optimal working condition was obtained. This method is simple and easy to implement and can be extended to processes where the target is identifiable in a transparent reactor and where both mixing and heat transfer effects need to be quantified.

Studies on thermal energy storage system with ceramic honeycomb channels

In this study, a ceramic-based sensible thermal energy storage system is analysed using analytical and numerical models, and the results subsequently validated with laboratory experiments. Corundum mullite monoliths are used as the storage material which is thermally cycled using compressed air as the heat transfer fluid (HTF). Here, hexagonal channels in the monolith allow direct contact heat exchange between the air and the storage material. The storage system is composed of a packed bed of ceramic blocks having honeycomb flow passages. However, unlike the case of packed bed of unconsolidated storage materials, the arrangement of these blocks plays a crucial role in the performance of the storage system. In this study, we try to bring out this crucial aspect of block arrangement and the effect it has on the flow characteristics and hence on the charging and discharging dynamics of the system. To understand the effect of ceramic block configuration in a storage system, two separate block arrangements are explored: parallel flow arrangement and series flow arrangement subjected to a single charging and discharging cycle. The storage systems are subjected to a constant temperature front of 450 K during charging cycle and 300 K during discharging cycle. Firstly, an analytical model is developed for airflow through a ceramic channel that predicts the temperature-time history of the storage system and the HTF at the system outlet. Subsequently, a full CFD analysis for coupled flow and conjugate heat transfer is performed on the two block arrangements, considering the actual three-dimensional ceramic block geometry with honeycomb channels. To validate the results of the analytical and numerical models, experimental testing is performed on a laboratory scale test setup. Finally, the effect of air velocity on the charging-discharging dynamics for the two arrangements of the ceramic blocks is also discussed. It is observed that the series flow arrangement charges and discharges the storage system 1.5 times faster than the parallel flow arrangement of the ceramic blocks but with four times higher pressure drop.