Ceramics and ceramic matrix composites for heat exchangers in advanced thermal systems—A review

In air-conditioning and energy-recovery applications, heat exchangers are very important to the overall efficiency, cost, and size of the system. Current heat exchanger designs rely heavily on fin-and-tube or plate heat exchanger designs, often constructed using copper and aluminum. Recent developments in material science—in particular, advances in ceramics and ceramic matrix composites—open opportunities for new heat exchanger designs. Some research directed toward using these materials for heat exchangers in other applications has been reported; however, there has not been a comprehensive study of the use of these emerging materials in both conventional HVAC&R systems and emerging energy technologies. This review reports the current state-of-the-art of ceramic materials for use in a variety of heat transfer systems.     

A new frontier of nanofluid research – Application of nanofluids in heat pipes

Nanofluid is a new kind of working fluid with special properties to enhance the heat transfer of heat pipes. This paper reviews and summarizes the research done on heat pipes using nanofluids as working fluids in recent years. The effect of characteristics and mass concentrations of nanoparticles on the thermal performance in various kinds of heat pipes with different base fluids under various operating conditions have been discussed. The mechanism of enhancement or degradation of heat transfer utilizing nanofluids in the investigated heat pipes has been explained. The paper discusses the relative reduction of the total heat resistance for various heat pipes with nanofluids in comparison with the existing ones and also presents a perspective on possible future research applications.     

Enhancement of heat transfer using nanofluids—An overview

A colloidal mixture of nano-sized particles in a base fluid, called nanofluids, tremendously enhances the heat transfer characteristics of the original fluid, and is ideally suited for practical applications due to its marvelous characteristics. This article addresses the unique features of nanofluids, such as enhancement of heat transfer, improvement in thermal conductivity, increase in surface volume ratio, Brownian motion, thermophoresis, etc. In addition, the article summarizes the recent research in experimental and theoretical studies on forced and free convective heat transfer in nanofluids, their thermo-physical properties and their applications, and identifies the challenges and opportunities for future research.     

Mechanisms proposed through experimental investigations on thermophysical properties and forced convective heat transfer characteristics of various nanofluids – A review

Experimental investigations on thermophysical properties and forced convective heat transfer characteristics of various nanofluids are reviewed and the mechanisms proposed for the alteration in their values or characteristics due to the addition of nanoparticles are summarized in this review. A comprehensive review on the experimental works on specific application of nanofluids is also presented. As the literature in this area is spread over a span of two decades, this review could be useful for researchers to have an accurate screening of wide range of experimental investigations on thermophysical properties, forced convective heat transfer characteristics, the mechanisms involved and applications of various nanofluids.     

Determining parameters of heat exchangers for heat recovery in a Cu–Cl thermochemical hydrogen production cycle

A heat exchanger is a device built for efficient heat transfer from one medium to another. Shell and tube heat exchangers are separated wall heat exchangers and are commonly used in the nuclear and process industry. The CuCl cycle is used to thermally crack water in to H₂ and O₂. The present study presents the heat exchanger thermal design using analysis of variance for heat recovery from oxygen at 500 °C, coming from the molten salt reactor. Polynomial regressions in terms of the amount of chlorine in the oxygen, the mass flow rate on the tube side, and the shell's outlet temperature are estimated for various exchanger parameters and the results are compared with the bell Delaware method. Based on energy and exergy analysis, this study also discusses the best possible path for the recovered heat from oxygen. Optimal heat exchanger parameters are estimated by Design-Expert^® Stat-Ease for most effective heat recovery.     

Numerical investigation of heat transfer and pressure drop characteristics of tube–fin heat exchangers in ice slurry HVAC system

This paper analyzes the heat transfer and pressure drop characteristics of a tube–fin heat exchanger in ice slurry HVAC system. Ice slurry is a suspension of crystallized water based - ice solution with a freezing point depressant like ethylene glycol. The ice- slurry is pumpable, hence it is also called pumpable ice. The composition of ice slurry considered for analysis is 14% ice fraction, 16% ethylene glycol, and 70% water by volume. It is deduced that the ice slurry HVAC system results in 7.4% increase in temperature drop over the conventional chilled water system The latent heat absorbed by ice slurry on melting makes it an attractive choice for achieving high degree of cooling. The numerical analysis was conducted by simulating the ice slurry tube flow region and air flow region of tube–fin heat exchanger in the air-handling unit of HVAC system. For the simulation six different louver patterns with 10 to 55 louver angle were considered. The design of the tube–fin heat exchanger for optimal heat transfer and pressure drop characteristics was also determined with the optimization parameter like louver angle, fin pitch, and ice slurry flow velocity.     

Generator absorber heat exchange based absorption cycle—A review

GAX based absorption cooling systems have been investigated in recent years by various groups across the world due to their advantage of offering a higher performance compared to that of the conventional ammonia–water absorption systems. In this paper, a comprehensive review of several different GAX cycle configurations has been explained in detail. The choice of working fluids and the performance of the GAX cycle in terms of coefficient of performance and temperature lift are also presented. The study reveals an improvement in the COP of about 10–20%, 20–30% and 30–40% in absorber heat recovery cycle, simple GAX and branched GAX cycle respectively, than that of a conventional single effect system for the same set of operating conditions. The importance of the GAX cycle with respect to the current energy scenario is also highlighted.     

Airside performance of fin-and-tube heat exchangers in dehumidifying conditions – Data with larger diameter

This study presents the airside performance of the fin-and-tube heat exchangers having plain fin geometry with a larger diameter tube (D_c = 15.88 mm) under dehumidifying condition. A total of nine samples of heat exchangers subject to change of the number of tube row and fin pitch are made and tested. It is found that the effect of fin pitch on the sensible j factor is, in general, diminished with the rise of tube row. However, there is a unique characteristic of fin pitch at a shallow tube row, the heat transfer performance is first increased at a wider pitch but a further increase of fin pitch lead to a falloff of heat transfer performance due to interactions amid flow development and bypass flow. The influence of tube row on the airside performance is rather small for both heat transfer and frictional characteristics at a fin pitch of 2.1 mm and when the Reynolds number is less than 4000. A slight deviation of this effect is encountered when fin pitch is increased to 2.54 mm or 3.1 mm due to condensate adhered phenomena.     

Energy autonomy in sustainable communities—A review of key issues

Recent years have seen the successful development and deployment of a range of small scale renewable energy systems. Driven in part by improving technical capability and by ambitious carbon emissions reduction targets, there has been the beginning of a shift towards a more distributed energy generation model, capable of delivering a range of potential benefits, but also presenting a number of social and technical challenges.One area of society where the benefits can be seen as being both highly applicable and highly relevant is at the community level and at this scale in particular, increased levels of energy autonomy can deliver a host of social, financial and environmental benefits. Therefore, the concept of energy autonomy is widely regarded as an effective tool in the push towards sustainable development, with ‘sustainable communities' often highlighted as particularly relevant for applying its principles.Given its significance and its broad interdisciplinary relevance, the issue, and the challenges it poses, has been the subject of a significant level of research interest in recent years. This study therefore presents a state of the art review of current research relating to energy autonomy in sustainable communities and identifies a number of central issues which are regarded as being of critical importance. Demand Side Management is identified as one particular area in need of further research and development, along with the need for receptive social, political and regulatory environments.     

A novel ‘‘partial bypass” concept to augment the performance of air-cooled heat exchangers

This study proposes a novel ‘‘partial bypass” concept which facilitates the heat transfer augmentation without the pumping power penalty. The basic idea is to introduce bypass cold stream which bypasses the front part of the heat exchanger and mixes with the hot stream across the front part of the heat exchanger, and the mixed stream flows through the rear part of the heat exchanger. Through this concept, heat transfer performance at the front part is slightly reduced but the rear part is substantially increased. The concept can be implemented with either maintaining the same total flowrate, heat transfer rate but accompanies with a much lower pressure drop or it can be applied with the same pumping power but accompanies with a much higher heat transfer rate. It is found that the “partial bypass” concept is especially fruitful for a heat exchanger with a comparatively large number of transfer unit (NTU). The concept is especially helpful in the design of a deep tube bank operated at lower frontal velocity. For validating the proposed concept, the calculation is also compared with the experimental measurements, and good agreement amid the calculations and measurements are reported.     

Ejector enhanced vapor compression refrigeration and heat pump systems—A review

The present paper provides a literature review on two-phase ejectors and their applications in vapor compression refrigeration and heat pump systems. Geometry, operation and modeling of ejector, and effects of various operating and geometric parameters, and refrigerant varieties on the ejector performances as well as performance characteristics of both subcritical and transcritical vapor compression systems with various cycle configurations are well-summarized. Moreover, system optimal operation and control to get maximum performance by using ejector as an expansion device are also discussed. However, a lot of research work still needs to be done for large-scale applications in industry and for the replacement/modification of conventional refrigeration and heat pump machines. Favorable performance improvement along with several advantages in installation, operation and control with ejector stimulates the commercialization of ejector enhanced refrigeration and heat pump systems and hoping this contribution will be useful for any newcomer in this field of technology.     

Mitigation of ice crystallization fouling in stationary and circulating liquid–solid fluidized bed heat exchangers

Liquid–solid fluidized bed heat exchangers are attractive ice crystallizers since they are able to mitigate ice crystallization fouling and exhibit high heat transfer coefficients. Experiments show that the fouling removal ability of stationary fluidized beds increases with decreasing bed voidage (95–80%) and increasing particle size (2–4 mm). The removal of ice crystallization fouling appears to be more effective in circulating fluidized beds, especially at high circulation rates. Fouling removal is realized by both particle–wall collisions and pressure fronts induced by particle–particle collisions. A comparison between ice crystallization experiments and impact characteristics shows that the removal rate is proportional to the impulse exerted on the wall. A model based on these phenomena is discussed and predicts the transition temperature difference for ice crystallization fouling in both stationary and circulating fluidized beds with an average absolute error of 9.2%.     

Nucleate pool boiling heat transfer of TiO2–R141b nanofluids

Nucleate pool boiling heat transfer of a refrigerant-based-nanofluid was investigated at different nanoparticle concentrations and pressures. TiO₂ nanoparticles were mixed with the refrigerant HCFC 141b at 0.01, 0.03 and 0.05 vol%. The experiment was performed using a cylindrical copper tube as a boiling surface. Pool boiling experiments of nanofluid were conducted and compared with that of the base refrigerant. The results indicate that the nucleate pool boiling heat transfer deteriorated with increasing particle concentrations, especially at high heat fluxes. At 0.05 vol%, the boiling heat transfer curves were suppressed. At high pressures of 400 and 500 kPa, the boiling heat transfer coefficient at a specific excess temperature was almost the same.     

Bubble generation in micro-volumes of “nanofluids”

We report experimental investigation of a transparent flat mini evaporator heated by laser beam. The influence of non-absorbing and absorbing nanoparticles immersed in pure water, and heat absorbing fluid on the heat transfer intensification was analyzed. Nanoparticles may initiate vaporization and boiling of fluid at low heat input. Providing specific task and conditions the nanoparticles may be used in passive or active modes. Passive mode assumes that nanoparticles do not generate thermal energy and improve bubble nucleation conditions due to additional nucleation of the fluid, thus decreasing boiling/vaporization temperature thresholds. Active mode assumes that nanoparticles act as converters of optical energy into thermal one.     

Analysis of the process characteristics of an absorption heat transformer with compact heat exchangers and the mixture TFE–E181

In the project described in this paper an experimental rig for a one-stage absorption heat transformer was designed and constructed. One aim of the project was to reduce the investment costs for the apparatus. This incorporates new and less expensive compact brazed plate heat exchangers for generator, evaporator, condenser and solution heat exchanger. The absorber was designed as a helical coil pipe absorber, where the weak solution trickles down as a falling film outside of the coil. The tests of the equipment involved measurements using a mixture of trifluorethanol (TFE) and tetraethyleneglycoldimethylether (E181). The process characteristics were investigated for different temperatures of the rich solution leaving the absorber. Experimental results are presented and compared with the results of a computer simulation model. Additionally the model was used to compare the COP of the heat transformation process with the mixtures lithium bromide–water (LiBr–H₂O) and ammonia–water (NH₃–H₂O). Furthermore, the overall heat and mass transfer coefficients for the plate heat exchangers and the falling film absorber were evaluated and compared with those of shell and tube heat exchangers.     

Airside performance of herringbone wavy fin-and-tube heat exchangers – data with larger diameter tube

This study examines the airside performance of the wavy fin-and-tube heat exchangers having a larger diameter tube (D_c = 16.59 mm) with the tube row ranging from 1 to 16. It is found that the effect of tube row on the heat transfer performance is quite significant, and the heat transfer performance deteriorates with the rise of tube row. The performance drop is especially pronounced at the low Reynolds number region. Actually more than 85% drop of heat transfer performance is seen for F_p ～ 1.7 mm as the row number is increased from 1 to 16. Upon the influence of tube row on the frictional performance, an unexpected row dependence of the friction factor is encountered. The effect of fin pitch on the airside performance is comparatively small for N = 1 or N = 2. However, a notable drop of heat transfer performance is seen when the number of tube row is increased, and normally higher heat transfer and frictional performance is associated with that of the larger fin pitch.     

Conflation of ε-Ntu and EGM design methods for heat exchangers with uniform wall temperature

This paper conflates two heat exchanger design approaches – the ε-N_tu (effectiveness–number of transfer units) and the EGM (entropy generation minimization) – focusing on heat exchangers with uniform wall temperature, i.e. condensers and evaporators. An algebraic formulation which expresses the dimensionless rate of entropy generation as a function of the heat exchanger geometry (number of transfer units), the thermal-hydraulic characteristics (friction factor and Colburn j-factor), and the operating conditions (heat transfer duty, core velocity, surface temperature, and fluid properties) is derived. It is shown that there does exist a particular number of transfer units which minimizes the dimensionless rate of entropy generation. An algebraic expression for the optimum heat exchanger effectiveness, based on the working conditions, heat exchanger geometry and fluid properties, is also presented. The theoretical analysis led to the conclusion that a high effectiveness heat exchanger design does not necessarily provide the best thermal-hydraulic performance.     

A dispersion model for predicting the heat transfer performance of TiO2–water nanofluids under a laminar flow regime

Nanofluids are a suspension of particles with ultrafine size in a conventional base fluid that increases the heat transfer performance of the original base fluid. They show higher thermal performance than base fluids especially in terms of the thermal conductivity and heat transfer coefficient. During the last decade, many studies have been carried out on the heat transfer and flow characteristics of nanofluids, both experimentally and theoretically. The purpose of this article is to propose a dispersion model for predicting the heat transfer coefficient of nanofluids under laminar flow conditions. TiO₂ nanoparticles dispersed in water with various volume fractions and flowing in a horizontal straight tube under constant wall heat flux were used. In addition, the predicted values were compared with the experimental data from He et al. [14]. In the present study, the results show that the proposed model can be used to predict the heat transfer behaviour of nanofluids with reasonable accuracy. Moreover, the results also indicate that the predicted values of the heat transfer coefficient obtained from the present model differ from those obtained by using the Li and Xuan equation by about 3.5% at a particle volume fraction of 2.0%.     

Advances in recombinant battery separator mat (RBSM) separators for lead-acid batteries—a review

Microglass separators have been used in lead-acid batteries for more than 20 years with excellent results. This type of separator (known as recombinant battery separator mat (RBSM)) has allowed valve-regulated lead-acid (VRLA) battery technology to become a commercial reality. When the concept of the VRLA battery was developed, the requirements of the RBSM separator were not fully known nor appreciated. In many cases, the direction charted for the separator has not been the most beneficial path to follow for separator performance and battery life. In some cases, such as the density of the separator media, experience has shown that the most correct path (low density) does not give rise to long battery life. As VRLA battery technology matures, greater pressure on cost and quality has arisen, especially with the proposed transition to 42 V automotive applications. This paper reviews some of the advances and changes in the RBSM separator made over the last 20 years, and provides some thoughts on future directions for this essential component of the VRLA battery.