Abstract

Researches about comprehensive, systemic and multi-level assessment methods are limit. Therefore, a comprehensive assessment method of heat exchangers based on extension theory is proposed. Heat transfer, pressure drop, mechanism, safety and reliability, economy, structure, mechanism and production performance are considered and evaluated in detail. By using this method, the comprehensive performances of the shell-tube and printed circuit heat exchangers as the intermediate heat exchanger of the accelerator driven sub-critical system are evaluated for comparison. The results show that the economy performance and pressure drop for the shell-tube heat exchanger attain the highest grades because the processing technology is mature and flow velocity is lower. However, the structure performance parameter is the lowest level due to its large volume and mass. For the printed circuit heat exchanger, the safety and reliability performance attains the lowest grade due to its easily blocked mini-channels and the manufactured thin plate. However, the mechanism performance shows the highest grade. In addition, the pressure drop performance also attains the highest grade due to its large cross-sectional area and low flow velocity. In conclusion, the comprehensive performance of the printed circuit heat exchanger is theoretically better than the shell-tube heat exchanger.

Introduction

As a clean and renewable source of energy, nuclear energy has the unique advantage and development potential on reducing the emissions of greenhouse gases and mitigating the pressure of energy transmission effectively [1,2]. Currently, how to dispose the nuclear waste has the constraint on the development of nuclear energy. Through the collision of proton beam generated by the accelerator with the spallation target of lead-bismuth eutectic (LBE), the accelerator driven sub-critical system (ADS) could drive and sustain the process of the nuclear transmutation, and reduce the life and radioactive property of nuclear wastes, and therefore is one of the effective resolutions [3]. In this system, LBE is adopted as not only the medium of the spallation target but also the coolant of the reactor due to its good thermal-physical properties [[4], [5], [6]]. The heat produced by the reactor is transferred from LBE to the working fluid of the power generating system through the intermediate heat exchanger (IHE), and the performance of the IHE plays significant impacts on the functions of the ADS and the power generating system. Therefore, systematic and reasonable performance evaluation on the IHE is of great significance.

At first, the performance assessment on the IHE with LBE is from the first principle of thermodynamics. Heat transfer rate, heat transfer coefficient and pressure drop are directly adopted to present heat transfer and flow characteristics of heat exchangers. For example, Ma et al. [7] experimentally investigated the effect of the straight and U-type tubes on the performance of the IHE with LBE. The heat transfer coefficients and pressure drops are compared between these two heat exchangers. Xi et al. [8] experimentally tested the performance of a shell-and-tube heat exchanger (STHE) with LBE and helium. The heat transfer rate, the overall heat transfer coefficient and pressure drop of the heat exchanger are obtained with increasing the Reynolds number and inlet temperature. In order to increase commonality, parts of research investigate the dimensionless performance parameters related to heat exchangers, such as Nusselt number, friction factor, effectiveness and number of transfer units (NTU). Chen et al. [9] numerically simulated a STHE with LBE and they utilized the overall heat transfer coefficient to compute the NTU and effectiveness of the heat exchanger for evaluation. Wang et al. [10] compared the effectiveness and friction factor of the STHE with LBE and helium with those concluded from correlations. Khan et al. [11] designed a compact heat exchanger with LBE and the heat transfer coefficient and Nusselt number are adopted to evaluate the performance of the heat exchanger. In order to fully consider the flow and heat transfer characteristics of the IHE, a comprehensive performance evaluation criterion (PEC) is introduced. Zhao et al. [12] changed straight tubes with twisted tubes for the IHE. The results show that the PEC of the twist-tube heat exchanger is improved by 1.2–1.7 times compared with the straight-tube one. Yang et al. [13] numerically studied the STHE with LBE and water, and the PEC is analyzed to optimize the inlet velocity of LBE.

Conclusions

In this study, through the analysis on matter element model, a comprehensive evaluation system of heat exchangers is built. The classical and joint domains are confirmed according to the databases and expert opinion. The weights of the performance indexes of the heat exchanger are set based on fuzzy analytic hierarchy process. The correlated function and evaluation grade are defined. At last, a comprehensive evaluation method of heat exchangers based on extension theory is proposed.

The SHTE and PCHE are chosen as the IHE with LBE and helium of the ADS for theoretical comparison. Although the flow channel in the SHTE is larger and that in the PCHE, the pressure drop performance for these two heat exchangers show the same grade. This is because the cross-sectional area of the PCHE is large and therefore the fluid flow velocity is low. The mechanism performance of the PCHE attains the highest grade because the plates are welded by the vacuum diffusion bonding. In conclusion, these two heat exchangers show the same evaluation grade. However, the comprehensive performance of the PCHE is theoretically better than that of the STHE due to the larger characteristic value. With the develop of the processing technology and the wide application of the PCHE, the production cycle is shortened and the manufacture and cost is deduced. In the future, the comprehensive performance of the PCHE would be even better.

Abstract

The heat transfer performance (HTP) of an improved elastic tube bundle (IETB) was analyzed using a bi-directional fluid-structure interaction calculation method. The vibration-enhanced HTP of heat exchangers with different inlet velocities (Uin) and row number N were studied. The results show that the amplitude gradually increases as the Uin increase with a maximum increase of 309.98%. The amplitude with fixed shell side length (FL-heat exchanger) is generally higher than that with variable shell side length (VL-heat exchanger) under different row numbers. The average amplitude of the three directions increased by 4.55%, 11.54%, and 7.41%, respectively. The heat transfer coefficient (h) is directly proportional to Uin, and the comprehensive HTP gradually decreases with the increase of Uin. With the increase in the row numbers, h generally showed a downward trend. For the VL-heat exchanger, h decreases as the row number increases, which increases from N=6 to N=9, and h decreases by 13.85%. For the FL-heat exchanger, h first increases and then gradually decreases. The comprehensive HTP also maintained the same trend. Compared VL-heat exchanger and FL-heat exchanger, reducing the spacing between tube rows increases heat transfer per unit volume, leading to improved comprehensive HTP of the IETB heat exchanger.

Introduction

Heat exchangers [1–3] are commonly used devices for exchanging energy in industries such as aerospace, chemical, and refrigeration production. When the traditional tubular heat exchanger works, its internal rigid heat transfer elements begin to vibrate under the impact of fluid, which can result in fatigue damage and decreased heat exchanger service life [4,5]. Shell and tube heat exchangers [6,7] are currently the most widely used in industrial production. The study of enhanced heat transfer in heat exchangers has become a hot topic of current research [8]. To reduce the influence of fluid shock, utilizing the vibration-enhanced heat transfer principle, an elastic tube bundle heat exchanger is proposed [9,10]. The material of the heat transfer element was replaced from steel to copper to achieve small amplitude vibration of the internal tube bundle at the low frequency, this improvement not only improves the heat transfer coefficient (h), but also uses the vibration deformation to remove the scale on the heat transfer surface, reduce the thermal resistance, and finally realize the composite heat transfer enhancement. However, due to the disadvantages such as poor comprehensive heat transfer performance (HTP) of elastic tube bundles, it is essential to improve the structure to obtain better comprehensive HTP.

Conclusion

Based on the bi-directional fluid-structure interaction calculation method, the effect of Uin and row number N on vibration and HTP of the IETB heat exchanger are studied. According to the results above, the key conclusions follow.

(1) The average amplitude of IETBs in different row numbers gradually increases with the rising of Uin. The vibration uniformity of the IETBs is better at low Uin. Besides, the difference in amplitude among different IETBs becomes larger and larger. And the amplitude in the FL-heat exchanger is generally higher than the VL-heat exchanger. For the VL-heat exchanger, when the row number is increased, the average amplitude decreases gradually. For the FL-heat exchanger, it decreases first and then increases.

(2) The average heat transfer coefficient ha/hva of IETBs is directly proportional to Uin and comprehensive HTP decreases gradually with the rising of Uin. With the increase of tube number i, the ha/hva presents a general trend of decline. And ha/hav in the FL-heat exchanger is generally greater than that VL-heat exchanger. In addition, the hva is always greater than h, which indicates that vibration facilitates the achievement of heat transfer enhancement.

Abstract

Molten salt is one of the most promising heat transfer and thermal energy storage medium for the third-generation solar thermal power generation, and the spiral-wound tube heat exchanger can improve the heat transfer performance of molten salt. In this paper, the flow and heat transfer characteristics of molten salt nanofluids in spiral-wound tubes are experimentally and numerically studied. Molten salt nanofluids and synthetic oil as the heat transfer fluids, the heat transfer performance of them in the spiral-wound tube heat exchanger were studied. The Wilson separation method was used to process the experimental data, and the heat transfer correlation of molten salt nanofluids is obtained through experiments. The heat transfer correlation can be in good agreement with the experimental data, the heat transfer correlation on the molten salt nanofluids side is  , Re = 4296–8348, Pr = 13–23, and the maximum error is 17.9%. A three-dimensional model of the spiral-wound tube heat exchanger is used to explore the distribution of the pressure, temperature and velocity fields of the molten salt nanofluids. The friction factor correlation on the molten salt nanofluids side is  , Re = 3536–17684, Pr = 13–23 and the maximum error is 12.65%. The heat transfer correlation and resistance correlation of molten salt nanofluids obtained in this paper can contribute to the design of heat exchanger for the third-generation solar thermal power.

Introduction

Solar thermal power generation technology has received extensive attention due to high efficiency and sustainability. Energy storage technology is an effective measure to solve the problem of intermittent solar power generation [[1], [2], [3]]. Molten salt can be used in the field of energy storage because of its large heat capacity, low viscosity, low vapor pressure, scholars around the world have paid extensive attention and carried out related research [[4], [5], [6]].

Liu et al. [7] conducted an experimental study on the turbulent heat transfer of molten salt in a circular tube, and obtained the heat transfer correlation of molten salt in the circular tube. Chen et al. [8] conducted an experimental study on the convective heat transfer of Hitec salt in three kinds of transversely corrugated tubes. He et al. [9] explored the convective heat transfer characteristics of ternary salts in shell and tube heat exchangers through experiments. Yang et al. [10] studied the flow and heat transfer characteristics of molten salt in trough solar collectors by numerical simulation. Chen et al. [11] carried out numerical simulation on the convective heat transfer of molten salt mixed in a unilaterally heated horizontal square tube. The results show that under the condition of non-uniform heating, the buoyancy force causes the core area of molten salt flow to be close to the heating surface to form eddy currents, which can enhance the heat transfer effect. Yang et al. [12] studied the heat transfer and flow performance of molten salt in the annular channel with helical coils by numerical simulation. The results show that adding helical coils can effectively enhance the heat transfer of molten salt in the annular channel, but the flow resistance also increased. Du et al. [13] designed a U-shaped tube for the experimental test of the heat transfer characteristics of molten salt in a shell-and-tube heat exchanger in transition flow, and fitted the heat transfer correlation of molten salt. Qiu et al. [14] proposed a baffle rod shell-and-tube heat exchanger configuration applied in the field of concentrated solar thermal power generation, using ternary salt and heat transfer oil as shell-side working fluids for experimental research. He et al. [15] studied the turbulent heat transfer characteristics of molten salt in shell-and-tube heat exchangers with Reynolds numbers ranging from 10,000 to 91,000 and 11,000 to 27,000 on the tube side and shell side, respectively. The comparison of available correlations for molten salt is in Table 1.

Conclusions

In this paper, the spiral-wound tube heat exchanger is taken as the research object, and the molten salt nanofluids and synthetic oil are used as heat transfer fluid to carry out experiments and numerical simulation studies respectively. The experiment was carried out with the salt-side temperature ranging from 573 K to 613 K, mass flow rate ranging from 0.6 to 1 kg/s. The corresponding Reynold’s number range is Re = 4296–8348. The flow heat transfer characteristics inside the heat exchanger, the following results are obtained:

1- Introduction

2- Methodology

3- Results and discussion

4- Conclusion

References

Abstract

This paper presents the effect of heat exchanger design on heat pump performance based on partial load operating conditions. 3-D numerical analysis was conducted to calculate face velocity profiles for each outdoor heat exchanger design (rectangular, cylindrical, and trapezoidal) in 10 different operating conditions. Heat exchanger circuits were modified considering heat exchanger face velocity distributions, and seasonal heat pump performances were calculated with modified heat exchanger design. The maximum seasonal performance enhancement of 7.07% was achieved with a modified heat exchanger design. Air-side flow maldistribution could affect significantly refrigerant path design and heat exchanger performance as well as system performance. The analysis results also revealed that smaller refrigerant circuits at the upper part of the heat exchanger interacting with higher air velocity could further enhance the annual system performance.

Introduction

Energy and environmental issues are critical problems due to rapid increases in energy systems around the world. The International Energy Agency has reported that energy systems such as air conditioning and heat pump systems account for almost 700 million metric tons of CO2 equivalent direct (7%–19%) and indirect emissions (74%) per year, which is responsible for global warming and ozone depletion. Direct emissions can be reduced by introducing low global warming potential (GWP) refrigerants, and indirect emissions can be controlled using a more efficient system [1]. The finned tube heat exchanger is widely used as a condenser or evaporator of heat pump systems, and it is one of the most important components of the systems and plays a significant role in the system size and performance. Therefore, it is important to design more efficient, lightweight, cost-effective, and low power consuming finned tube heat exchangers [2,3]. Heat exchanger performance degradations are associated with maldistribution of refrigerant and face air velocity distribution, particularly for downstream rows as airflow velocity decreases significantly on the lower side [4]. Although it is difficult to measure the air velocity profile experimentally [5], it strongly influences heat exchanger performance [6]. Non-uniform flow distribution has many disadvantages: produces wake in flow [7], changes flow pattern [8], decreases overall heat transfer coefficient [9], and reduces system capacity [10]. Consequently, it also increases pressure drop [11] across the heat exchanger hence increases required pumping power [12–15]. Performance deterioration due to non-uniform flow is not limited to finned tube heat exchangers with louver fins [9] and plate fins [16,17], but also occurs in concentric tube [14], and microchannel condensers of automotive air conditioning systems [18]. These examples highlighted the problem of non-uniform flow distribution in different applications. However, these studies are limited to rated load conditions for specific heat exchanger design. No study has been found in literature that covers face velocity distribution for outdoor heat exchanger under partial load operating conditions. Therefore, it is important to evaluate face velocity profiles under off-design condition.

1- Introduction

2- Experimental investigation

3- Procedure and Calculations

4- Results and discussion

5- Conclusion

References

Abstract

Nanofluids have attracted huge attention because of their effective physical and thermal properties. One of many applications of nanofluids is the enhancement of the thermal performance of heat exchangers. In the current study, an experimental investigation has been conducted for studying the effects of graphene nanofluids on the convective heat transfer in a vertical shell and tube heat exchanger. Graphene flakes were prepared using graphite foam that is derived from sugar as a raw material. The prepared Graphene flakes have been characterized using scanning electron microscopy, X-ray diffraction, atomic force microscopy, and Raman spectroscopy. The graphene nanofluid has been used in the tube side of the heat exchanger to enhance its heat transfer performance. Different parameters such as nanofluids’ concentration, flow rate and inlet temperature were studied and their effects on heat transfer coefficient and thermal efficiencies are discussed. The results show that using of graphene/water nanofluids enhances the thermal performance of the vertical shell and tube heat exchanger. A maximum increase in the heat transfer coefficient of 29% was achieved using 0.2% graphene/water nanofluids. Furthermore, the mean thermal efficiency of the heat exchanger was enhanced by 13.7% by using graphene/ water nanofluid.

Introduction

Heat exchangers are widely used by various types of industries to exchange heat between different fluids for waste heat recovery and utilities cost reduction. The thermal and physical properties of the heat transfer fluids are crucial factors that dedicate the efficiency of heat exchangers. During the last two decades, nanofluids have attracted a huge attention of researchers due to its enhanced thermal properties and flow characteristics [1]. These advantages make nanofluids promising heat transfer fluids to be used for heat transfer enhancement. Many experimental studies have been conducted for the evaluation of heat transfer characteristics of nanofluids in different types of heat exchangers [2–7]. These types of heat exchangers include plate heat exchangers [2], double pipe heat exchangers [3], and micro heat exchangers [6]. However, in open literature, similar studies on shell and tube heat exchangers are scarce. Thermal performance of a shell and tube heat exchanger was analytically investigated by Shahrul et al. using nanofluids of four different types of nanoparticles, Fe3O4, ZnO, TiO2, CuO, and Al2O3 [8].

Gas-gas plate heat exchanger is an important component to remove heat generated from electronic devices in the cabinet cooling system. The counter-flow plate heat exchanger is usually used due to its higher heat transfer performance than the cross-flow plate heat exchanger. However, in the cabinet cooling system, the overall dimensions for the heat exchanger is limited. Therefore, it is necessary to consider the overall dimensions of the system during heat exchanger design, but the traditional thermal design method of heat exchanger doesn’t consider the effect of system parameters. In this paper, a cross-flow plate heat exchanger is proposed to improve the cooling performance of cabinet cooling system, and is compared with the counter-flow plate heat exchanger. The ε-NTU method and effectiveness-thermal resistance method are applied to evaluate the performance. It is found that for large width of cabinet cooling system, the system with cross-flow plate heat exchanger has higher cooling performance and lower thermal resistance than the system with counter-flow plate heat exchanger. When the width is 700 mm, the cooling capacity of the two systems are 176.13 W/K and 138.95 W/K, respectively. The dimensionless thermal resistance can characterize the irreversibility of the heat transfer at constant mass flow rate.

Introduction

Plate heat exchangers are widely used in energy consuming and handling industries [1]. Generally, plate heat exchangers are used for liquid–liquid heat exchange [2]. Imran et al. [3] optimized liquid–liquid plate heat exchanger. The overall dimensions of the heat exchanger were independently considered in their research. Miao et al. [4] numerically studied liquid–liquid plate heat exchangers. The heat exchanger was counter-flow plate heat exchanger. Two methods were utilized to get simulation results, respectively. The results of grey-box method were better. Kumar et al. [5] experimentally studied the liquid–liquid plate heat exchangers and obtained correlation based on their experimental data, in which the overall dimensions of the measured heat exchangers were constant. Yang et al. [6] obtained correlation based on their experimental data and empirical correlations in open literature. The overall dimensions of heat exchangers in their work were varied. Counter-flow plate heat exchanger also could be applied for two-phase heat exchange [7] and the heat exchangers in organic Rankine cycle could be counter-flow plate heat exchangers for better performance [8]. However, due to the primary heat transfer surfaces and light weight the plate heat exchanger was developed for gas–gas heat exchange [9] and even for liquid–gas heat exchange [10]. Cross-flow arrangement was convenient for liquid–gas plate heat exchanger [11]. High temperature gas–gas heat exchangers were mainly applied for the microturbine recuperated cycle system [12] and air preheater of the fossil fuel power system [13]. Recuperators for micro gas turbine was reviewed by Xiao et al. [14] recently. Wang et al. [15] improved the performance of air preheater by optimizing geometrical parameters of primary surface.

1. Introduction

2. Materials and methods

3. Results

4. Conclusions

Appendix A. Supplementary data

References

Abstract

The thermal system design strongly depends on material selection. Nanofluids offer design flexibility and finetuning of properties by incorporation of nanoparticles in base fluids. This flexibility is guided by particle-particle communication, which may be beneficial in creating ballistic routes in heat transfer but also detrimental due to affecting nanofluid properties. The transition of nanofluids to industrial use requires application-based examinations. For this purpose, different types of nanofluids were investigated in this work in terms of their thermal efficiency in a flat plate solar collector (FPSC) and some figure-of-merits (FOMs), under laminar and turbulent flow conditions. Investigation of both aims at clarifying the correlation between FOMs and FPSC thermal efficiency, and further reporting on the validity of FOMs in assessing thermal efficiency. Results indicate that nanofluids’ eligibility as a heat transfer fluid depends on the flow condition, since a base fluid could outperform a nanofluid under turbulent flow. Nanofluid type and nanoparticle shape affects thermal performance, as suspensions of nanoplatelets/nanotubes in low concentrations (< 0.04 vol%/0.25 vol%) are shown to outperform certain spherical metal-oxide nanoparticles (< 3 vol%), according to some FOMs. It is shown that performance evaluation criteria (PEC), overall energetic efficiency, and energy ratio (ER) do not capture FPSC thermal efficiency trends, e.g., for graphene nanoplatelet nanofluid, as Mouromtseff number-based comparisons do for laminar and turbulent conditions. It must be highlighted that the FOM type to indicate thermal efficiency should be chosen depending on the application, and simultaneous consideration of thermal and hydrodynamic characteristics is required.

Introduction

In today’s world, the main emphasis in energy industry lies on the concept of “sustainability” in energy applications. Plenty of Rooms at the Bottom are available, as stated by Richard P. Feynmann [1], at the enhanced surface-to-volume ratio of nanoparticles which bring about unique characteristics to base fluids when mixed with nanoparticles. Nanofluids [2] or nano-enhanced Heat Transfer Fluids (ne-HTFs) have been under research in a variety of heat transfer-based systems. This has been by necessity due to the poor thermal characteristics of most conventional HTFs, e.g., water, ethylene glycol, mineral oils, brines, etc. compared to solids. This fact highlighted the need for their replacement by ne-HTFs as a potential way to improve thermal performance of solar collectors, electronic cooling systems, nuclear reactor cooling schemes, refrigerators, and so forth.

Nanofluids thermal conductivity and heat transfer coefficient have been among the most investigated characteristics [3–5] in determining ne-HTFs’ potential in various heating and cooling processes. Hydrodynamic and colloid state points of view show per contra that high viscosity of nanofluids cause increments in pressure drop, pumping power, operation cost, and nanoparticle sedimentation concerns to increase; as well as sometimes thermal performances no better than HTFs in turbulent flow and fully developed flow conditions [3,6–8]. Although the convective heat transfer coefficient and Nusselt number are important performance indicators, various types of figure-of-merits (FOMs) have been proposed to compare the heat transfer characteristics of HTFs. One of the most earliest FOM definition is the Mouromtseff number (Mo) [9] that has been used to compare heat transfer capabilities of HTFs in laminar and turbulent flow conditions. Another FOM is defined as the ratio of viscosity enhancement and thermal conductivity enhancement coefficients [10], valid in the case of fully developed laminar flow. Sekrani et al. [11] suggested two different types of FOMs to compare the heat transfer capability of nanofluids as the performance evaluation criterion, i.e., PEC, [12] and overall energetic efficiency.

1. Introduction

2. Experimental apparatus and procedure

3. Modeling and calculation methodology

4. Experimental results and discussion

5. Conclusion

Acknowledgments

References

Abstract

Freeze concentration (FC) by suspension crystallization is a complex process due to the combination of a scrapedsurface heat exchanger, crystallizer, and wash column. Following the study of a three-in-one structure of a multipass FC published earlier this year, modeling and experiments of the heat and mass transfer of this freeze concentrator are presented in this paper. The experimental assessment of the system performance, including the measured values of the heat transfer coefficient, ice production rate, and energy efficiency as well as their correlation with the concentration ratio and partition coefficient are presented in this article.

Introduction

Freeze concentration (FC) is a nonthermal processing technology of liquid food, in which a portion of the water in the aqueous solution is frozen and converted into relatively pure ice crystals, after which it is removed from the liquid phase to concentrate the remaining solution. It can be used to concentrate or pre-concentrate heat-sensitive aqueous solutions, such as milk (Habib and Farid, 2007; Sanchez et al., 2011), fresh fruit juices (Orellana-Palma et al., 2017; Petzold et al., 2013; Bayindirli et al., 1993), other liquid foods (Moreno et al., 2014a), and biological solutions (Moreno et al., 2014b). Studies have shown that compared with evaporation concentration, FC has the advantage of producing less thermal denaturation of the solution, and thus, it can better maintain the original flavor, nutrition, and color of liquid foods (Benedetti et al., 2015; Moreno et al., 2014c; Miyawaki et al., 2016a). Moreover, the latent heat of water freezing is almost one seventh of the latent heat of water evaporation (Lide and Haynes, 2010), which offers potential for energy savings for de-watering of aqueous solutions.

Ice crystals can be formed from aqueous solutions in two ways: progressive crystallization (Miyawaki et al., 2005, 2016b; Zambrano et al., 2018) and suspension crystallization (Huige and Thijssen, 1972; Qin et al., 2007). In the former, the water freezes on the cooling surface, forming an ice layer progressively and a concentrated liquid phase. Thus, it is also known as layer crystallization. This method of FC is called progressive FC. When the ice layer extends to the entire vessel to form an ice block, it is known as block FC (Moreno et al., 2014c; Zambrano et al., 2018). The advantage of this technology is the low equipment cost and simple operation management. However, the ice layer has a poor heat transfer coefficient of less than 0.1 kW m 2 K 1 (Qin et al., 2003a; Pronk et al., 2010; Hasan et al., 2017), and a huge cooling surface area is required for practical applications. In addition, the ice layer tends to entrain liquid sacs and causes severe solute loss (Miyawaki et al., 2016a, 2016b; Samsuri et al., 2015).

Targeting the heat transfer performance of geothermal pile-foundation heat exchanger in ground source heat pump system (GSHP), the physical models of 3-U pile-foundation heat exchanger and heat exchanger group were established. CFD software was used to simulate heat transfer processes and heat transfer performance was analyzed both in cooling and heating mode. The simulation results indicated that the higher thermal conductivity of pile-foundation heat exchanger contributed to the higher heat transfer efficiency than soil. Heat transfer flux per meter of the pile-foundation heat exchanger gradually decreased with time went on. After operating for ten years, the average soil temperature increased by 2.96 K in non-equilibrium condition and decreased by 0.61 K in equilibrium condition. The equilibrium condition of cooling and heating load was beneficial to operation system’s safety and efficiency. The experimental values of temperature differences were 2.2 K, 2.5 K and 3.5 K, and the heat transfer flux were stable at 58.1 W·m − 1, 65.9 W·m − 1 and 46.2 W·m − 1 in three schemes separately. The maximum difference value was 8.4% for temperature difference between experiment and simulation. The simulation results corresponded well with experimental data, indicating the reliability of simulation. The study results were approximate to the actual situation and can be used as theoretical basis for design and application of pile-foundation heat exchanger in GSHP system.

Introduction

With the development of society and improvement of people’s living standards, energy consumption has been growing apace. Nowadays, urban heating has been accounting for the largest proportion of the total building energy consumption in China [1]. For the purpose of saving energy and protecting environment, geothermal energy, as one of renewable energies, has attracted increasing interest around the world. One of the main technologies of using geothermal energy is ground source heat pump system (GSHP), which has been widely studied and applied since the early 20th century. In contrast to traditional air conditioning system, GSHP system has a lot of significant advantages. By extracting heat into and injecting heat from soil, GSHP system can operate stably by avoiding the impact of weather changes on system performance, meeting the requirements of sustainable devel-opment strategy. Without material exchanges with atmosphere, the pollution of waste heat, vapor and noise can be reduced. Moreover, GSHP system has a great advantage in investment and maintenance costs [2]. However, the disadvantages of GSHP system cannot be ignored neither. To invest a new system, a large area of land is needed and drilling holes costs additional investment. In 1994, steel pipes in pile foundation were applied to buildings for the first time, and then the concept of pile-foundation heat exchanger was proposed [3]. Consequently, geothermal pile-foundation heat exchangers in GSHP system with pipes laying inside the foundation and fixing via concrete are applied, contributing to declining outlays, saving ground area and reducing thermal interference between heat exchanger [4]. Compared with horizontal pile-foundation heat exchanger, vertical heat exchangers had better performance in heat transfer efficiency and land consumption [5].

This work presents an experimental and theoretical evaluation of the thermal performance of a square straight diffusion-bonded stainless steel compact heat exchanger. A one-dimensional steady-state thermal model was proposed to predict the thermal characteristics of the heat exchanger. To validate the model and to study the thermal behavior of the heat exchanger, an experimental test apparatus was developed. The heat exchanger was tested in several combinations of Reynolds ranging from 2600 to 7500, representing transition to turbulent regimes. The temperatures were varied from 70 C to 80 C for the water and from 25 C to 42 C for the air, at the inlet of the heat exchanger, respectively. A good agreement between the experimental data and the analytical model was obtained.

Introduction

In the petroleum and process industries, shell and tube or variations represent about 35% of the total of the heat exchangers used, being by far the most popular technology [1,2]. Although these types of heat exchangers are reliable and robust, their large volumes and footprint area make them not appropriate to be used in some applications [1,3]. On the other hand, compact heat exchangers are highly efficient, as their main characteristic is their large heat transfer surface area for a fixed volume. Although these devices have evolved considerably lately to new efficient solutions, they still deserve a great deal of research around the world. Compact heat exchangers have been developed for applications where requirements of small weight and space are mandatory, as encountered in aerospace, naval and automotive fields. In many heat exchangers especially the compact ones, hot and/or cold streams may flow through non-circular cross-section ducts, i.e., triangular or rectangular, among other geometries. The lengths of these ducts are usually small. The equipment may operate in several regimes, varying from laminar to turbulent. Advanced heat exchangers, like the printed circuit (PCHE), are compact devices characterized by a large heat transfer surface to volume ratio, which presents high effectiveness and low terminal temperature difference. Generally, the compact heat exchanger is fabricated from a large number of plates with channels, chemically etched or water–jet machined [4,5]. After a stacking process, a diffusion bonding technique is applied for fabricating the cores.

1- Introduction

2- Case of study

3- Regression models description

4- Experimental set up

5- Experiment results

6- Conclusions and future work

References

Abstract

In recent years, the use of renewable energies has been promoted in most of developed countries due to the climate change threat. In this scenario, the importance of geothermal installations has increased. This paper focuses on a heat exchanger present on a geothermal installation. The main aim is to achieve an accurate prediction system using the previous readings of some of the sensors located along the heat exchanger. Different time series modeling techniques were applied obtaining satisfactory results in the prediction of the heat exchanger state during one year. This prediction is made 1 h, 3 h and 6 h in advance. Also, a strong correlation between the sensor readings is concluded, offering the possibility to dispense some of them.

Introduction

In last decades, the world is facing fast changes in terms of economic global instability, increase in competitiveness and climate change [1]. Under these circumstances, the rising price of raw materials and a high fossil fuels dependency, with the corresponding environment damage, led to an intensification in the use of alternative energy resources [2]. The supply of renewable energies is unlimited, and its use is relatively clean. For these reasons, during last decades, many governments have promoted policies that encourage the use of these kind of energies [3]. Usually, solar and wind are the most used alternative energies, and therefore, their technologies have experienced an intense development. However, nowadays, the field of renewable energies have presented significant advances in other disciplines such as geothermal or ocean energies [4]. These advances are aimed at narrowing the implantation costs and increasing the efficiency. Some researches are focused on developing new methods or optimizing existing ones in energy installations [5]. In some systems, different renewable energies like geothermal and solar are combined to improve the efficiency [6]. This paper focuses on a geothermal energy installation. Geothermal is defined as the energy stored in form of heat inside the earth under the ground [7]. Different works estimate that the whole amount of heat flowing from inside the earth is around 42 × 1012 W [7]. Only the 2 % of this energy comes from the crust, the 82 % comes from the mantle due to the decay of radioactive isotopes of uranium, potassium and thorium and the remaining percentage is generated at the core. Despite the vast amount of energy, its use is limited to specific areas with proper geological conditions [8]. At the beginning of twenty first century, the use of geothermal energy to produce electricity had an installed capacity of almost 9 MW, while its use in non-electrical applications represented about 15 MW. The use of nonelectrical geothermal energy is extended in applications like heat pumps, bathing, space heating, green houses, aquaculture and industrial processes [7]. A heat pump is employed to provide thermal energy obtained from an external source to a facility [9]. The external source can be hot or cold, leading the first one to higher efficiency. In geothermal heat pumps, the thermal source is hot and the heat exchanger installation can be placed in horizontal or vertical configurations [10, 11].

1- Introduction

2- Triple tube exchanger advancements

3- Conclusions

References

Abstract

In the past decades have faced the challenges in meeting out the cooling demand due to the technological developments. More works are being carried out for optimum heat transfer in concentric heat exchangers, one among the double tube heat exchangers investigation is the triple tube heat exchangers (TTHE). Reported that the triple tube heat exchangers are more effective than double tube heat exchangers. In this paper, investigations carried out on triple tube heat exchangers are reviewed and the reports published by different researchers are critically analyzed in order to carry out the advanced research work on triple tube heat exchangers.

Introduction

The basic type of heat exchanger used in a wide range of applications is a double tube heat exchanger. Its working phenomenon is based on heat transfer between two pipes with cold and hot fluids. It has its significance in freezing, boiling drying, dairy, and pharmaceutical, food, pasteurization and chemical industries. In order to manage high-temperature differences, the heat exchange area should be increased, which could be possible only by increasing the heat exchanger’s length. TTHE solves this problem, by increasing the heat transfer area for the same length of the double tube heat exchanger. It has three concentric tubes specifically three compartments which are mentioned to be inner tube, inner annulus and outer annulus. It is needed to flow the target fluid whose temperature variations are of main concern for application fulfillment within the inner annulus for taking better advantage of the heat exchanger. The inner annulus has 2 heat exchange surfaces (inner tube outer surface and outer tube inner surface) that will increase the heat exchange area of heat exchanger marginally compared to double tube device (has just one heat exchange surface) which will increase the speed of heat exchange. It conjointly in terms will increase the potency of heat exchanger.

1- Introduction

2- Mathematical modeling

3- Data reduction

4- Results and discussion

5- Conclusions

References

Abstract

In this paper, an optimization method is proposed to improve the performance of a novel heat exchanger with latent heat thermal energy storage. Paraffin/expanded graphite composites were used as phase change material filled in annular tube, maintained contact with the shell and tube flow channels simultaneously. The heat transfer and flow characteristics of this novel heat exchanger were investigated in a previous study, which indicated that further research was required to enhance thermal transfer rates. Therefore, in this work, the thermal property of the material and geometric parameters of the heat exchanger, including compression density, tube diameter, and fins and baffle configuration, were studied numerically to improve the thermal behavior of the system. It was found that a composite with density of 600 kg/m3, a tube diameter of 19 mm, and an extended surface and baffle space of 100 mm, can be used for the final optimization result. The results show that, after optimization, the thermal conductivity of the phase change material can reach 5.16 W/m K and the latent thermal energy stored in system could be twice that previously obtained. The solidification rate can increase to a maximum of 50% while the shell side Nu/Pr1/3 can reach 29.98. The improved heat exchanger shows better energy storage capacity and working power.

Introduction

Currently, thermal energy storage (TES) plays an important role in overcoming the lack of balance between energy supply and demand. TES can be divided into three technologies: sensible, latent, and thermochemical. Latent heat thermal energy storage (LHTES) based on a phase change material (PCM) is considered to be a promising technique, because it has high energy storage density and low-fluctuation operating temperature [1]. Owing to the superior performance of the LHTES unit, it is employed in many fields, such as industrial waste heat recovery [2], solar energy utilization [3], thermal management of electron device [4], and building energy conservation [5]. A heat exchanger based on latent heat storage is the key of the LHTES unit. A cylindrical shell-and-tube heat exchanger is commonly used for heat storage system owing to its robust structure, easy maintenance, and high heat transfer rate [6]. Cano et al. [7] studied four types of PCMs filled in the shell side of the heat exchanger and the influence of the heat transfer flow rate, and different operation modes (watertight and countercurrent PCM flow) were evaluated. It was found that the shell-side PCM countercurrent flow system showed the best energy transfer performance. Hosseini et al. [8] investigated the melting and solidification performance of paraffin RT50 in the shell side of a shell-and-tube heat exchanger and found that by increasing the inlet water temperature from 70 C to 80 C, the charging and discharging efficiency of the heat exchanger rose from 81.1% to 88.4% and 79.7% to 81.4%, respectively. Kibria et al. [9] experimentally and numerically studied the latent heat storage system with shell and tube arrangement, where the PCM was filled in the shell and water passed through the tube. It was found that the influence of inlet temperature of the fluid on the solidification and melting of the PCM is considerable, whereas that of the mass flow rate is negligible. In spite of the great potential of the LHTES system, the low thermal conductivity of the PCM still limits its practical feasibility.