Thermal evaluation of coaxial deep borehole heat exchangers

This paper presents a performance study of deep borehole heat exchangers. The coaxial borehole heat exchanger (BHE) has been selected because for the present conditions it has a better performance than the conventional U-tube BHE. A numerical model has been developed to study the coaxial BHE. The model predictions are compared to detailed distributed temperature measurements obtained during a thermal response test. The model is found to accurately predict the behavior of a coaxial BHE. The influence of the flow direction of the mass flow is studied for BHEs in the range 200 m–500 m. A parametric performance study is then carried out for the coaxial case with different borehole depths, flow rates and collector properties. The results clearly show a significant increase in the system performance with depth. In addition, it is shown that with increasing borehole depth, the heat load that can be sustained by the BHE is significantly increased. An overall performance chart for coaxial BHEs for the depths of 300–1000 m is presented. The chart can be used as a guide when sizing deep BHE installations.     

Application of nanofluids in heat exchangers: A review

The purpose of this review summarizes the important published articles on the enhancement of the convection heat transfer in heat exchangers using nanofluids on two topics. The first section focuses on presenting the theoretical and experimental results for the effective thermal conductivity, viscosity and the Nusselt number reported by several authors. The second section concentrates on application of nanofluids in various types of heat exchangers: plate heat exchangers, shell and tube heat exchangers, compact heat exchangers and double pipe heat exchangers.     

Thermal performance evaluation of heat exchangers fitted with twisted-ring turbulators

Heat transfer, friction factor and thermal performance characteristics in a tube equipped with twisted-rings (TRs) are experimentally investigated. The experiments were conducted using TRs with three different width ratios (W/D = 0.05, 0.1 and 0.15) and three pitch ratios of (p/D = 1, 1.5 and 2) for Reynolds numbers ranging from 6000 to 20,000 using air as a test fluid. The typical circular rings (CRs) were also tested for an assessment. The experimental results reveal that most TRs yield lower Nusselt numbers and friction factor than CRs, except at the largest width ratio (W/D = 0.15) and the smallest pitch ratio (p/D = 1.0). In addition, Nusselt number and friction factor increase as width ratio increases and pitch ratio decreases. However, a maximum thermal performance factor is associated by TRs with the smallest width ratio and pitch ratio. The empirical correlations of the heat transfer (Nu) and friction factor (f) are also included in this paper.     

Heat transfer characteristics in double tube helical heat exchangers using nanofluids

In the present work a three-dimensional analysis is used to study the heat transfer characteristics of a double-tube helical heat exchangers using nanofluids under laminar flow conditions. CuO and TiO₂ nanoparticles with diameters of 24 nm dispersed in water with volume concentrations of 0.5–3 vol.% are used as the working fluid. The mass flow rate of the nanofluid from the inner tube was kept and the mass flow rate of the water from the annulus was set at either half, full, or double the value. The variations of the nanofluids and water temperatures, heat transfer rates and heat transfer coefficients along inner and outer tubes are shown in the paper. Effects of nanoparticles concentration level and of the Dean number on the heat transfer rates and heat transfer coefficients are presented. The results show that for 2% CuO nanoparticles in water and same mass flow rate in inner tube and annulus, the heat transfer rate of the nanofluid was approximately 14% greater than of pure water and the heat transfer rate of water from annulus than through the inner tube flowing nanofluids was approximately 19% greater than for the case which through the inner and outer tubes flow water. The results also show that the convective heat transfer coefficients of the nanofluids and water increased with increasing of the mass flow rate and with the Dean number. The results have been validated by comparison of simulations with the data computed by empirical equations.     

Heat transfer enhancement using hybrid nanofluids in spiral plate heat exchangers

A possible way to enhance the rate of heat transfer in the spiral plate heat exchanger (SPHE) is by employing hybrid nanofluids as its working medium. Hence, in the present work, effects of hybrid nanofluids on the thermal performance of SPHE has been investigated numerically. First, a countercurrent SPHE is designed and modeled. Later, simulation of SPHE has been carried out by employing conventional fluid , nanofluids , and hybrid nanofluids to investigate the heat transfer rates. Finally, the performance of SPHE using hybrid nanofluid is compared with that of using water and nanofluids. The heat transfer augmentation of approximately 16%‐27% with hybrid nanofluids of overall 4% nanoparticles volume concentration and 10%‐16% with 2% nanoparticles volume concentration is observed when compared with that of pure water. Therefore, it can be inferred that the application of hybrid nanofluids in SPHE seems to be one of the promising solutions for augmentation of its thermal performance.     

Thermal design of multi-stream heat exchangers

The thermal design of multi-stream heat exchangers of the plate and fin type is presented. Although originally used in low temperature processes, their application is extrapolated to above temperature processes and it is shown that, conceptually, multi-stream exchangers could replace whole heat recovery networks. The approach is based on the use of temperature vs. enthalpy diagrams or composite curves, which show how a multi-stream exchanger can be subdivided into block sections that correspond to enthalpy intervals and indicate the entry and exit points of the streams. A design methodology for plate and fin exchangers in countercurrent arrangement, characterized by the maximization of allowable pressure as a design objective is extended to the design of multi-fluid exchangers. The methodology uses a thermo-hydraulic model which relates pressure drop, heat transfer coefficient and exchanger volume. The potential applicability of the methodology is demonstrated on a case study.     

Thermal analysis on metal-foam filled heat exchangers. Part II: Tube heat exchangers

The forced convection heat transfer characteristics in high porosity open-cell metal-foam filled tube heat exchangers are analysed in this paper. The Brinkman-extended Darcy momentum model and two-equation heat transfer model for porous media are employed for the analysis of the heat transfer performance. The morphological effects of metal foams on overall heat transfer are examined. The optimal foam-area ratio for a metal-foam filled counter-flow tube-in-tube heat exchanger is predicted. The study shows that the thermal performance of a metal-foam heat exchanger can be superior to that of conventional finned tube heat exchangers.     

Thermal performance of flat-shaped heat pipes using nanofluids

Analytical models are utilized to investigate the thermal performance of rectangular and disk-shaped heat pipes using nanofluids. The liquid pressure, liquid velocity profile, temperature distribution of the heat pipe wall, temperature gradient along the heat pipe, thermal resistance and maximum heat load are obtained for the flat-shaped heat pipes utilizing a nanofluid as the working fluid. The flat-shaped heat pipe’s thermal performance using a nanofluid is substantially enhanced compared with one using a regular fluid. The nanoparticles presence within the working fluid results in a decrease in the thermal resistance and an increase in the maximum heat load capacity of the flat-shaped heat pipe. The existence of an optimum nanoparticle concentration level and wick thickness in maximizing the heat removal capability of the flat-shaped heat pipe was established.     

Thermal performance of inclined grooved heat pipes using nanofluids

An experimental study was performed to investigate the thermal performance of an inclined miniature grooved heat pipe using water-based CuO nanofluid as the working fluid. This study focused mainly on the effects of the inclination angle and the operating pressure on the heat transfer of the heat pipe using the nanofluid with the mass concentration of CuO nanoparticles of 1.0 wt%. The experiment was performed at three steady sub-atmospheric pressures. Experimental results show that the inclination angle has a strong effect on the heat transfer performance of heat pipes using both water and the nanofluid. The inclination angle of 45° corresponds to the best thermal performance for heat pipes using both water and the nanofluid. The present investigation indicates that the thermal performance of an inclined miniature grooved heat pipe can be strengthened by using CuO nanofluid.     

Thermal wave effects on heat transfer enhancement in nanofluids suspensions

The heat transfer enhancement revealed experimentally in nanofluids suspensions is being investigated theoretically at the macro-scale level aiming at explaining the possible mechanisms that lead to such impressive experimental results. In particular, while the possibility that thermal wave effects via hyperbolic heat conduction could have explain the excessively improved effective thermal conductivity of the suspension the comparison with experimental results rules-out this explanation.     

Experimental evaluation of solar thermosyphons with heat exchangers

The advantages of solar thermosyphons in terms of simplicity, reliability and cost have long been recognized. Recent studies have also shown their thermal performance to be comparable with that of equivalent active systems. When pump power is considered, the energy savings of domestic hot water thermosyphons can be significantly superior to active systems. In spite of these advantages, use of solar thermosyphons in the United States is almost negligible compared to their widespread use in other countries. A major limitation to the use of thermosyphons in the United States is lack of effective, reliable freeze protection. One technique for reliable, passive freeze protection is to use a heat exchanger in the storage tank and a nonfreezing fluid in the collector. Previous analytical work indicates that the performance penalty for these systems with practical-sized heat exchangers may be small enough to make these systems economically feasible. A full-scale, residential-size test facility has been constructed for testing this concept and validating the theoretical models. This paper describes results of testing comparing the performance of a horizontal tank with and without heat exchanger to a baseline case of a vertical tank without heat exchanger. An analytical expression for a “heat exchanger penalty factor” for these systems is derived and compared with the experimental results.     

Heat transfer and entropy analysis of three different types of heat exchangers operated with nanofluids

The development of nanotechnology has witnessed an emergence of new generation of heat transfer fluids known as nanofluids. Nanofluids are used as coolants which provide excellent thermal performance in shell and tube heat exchangers. However, the viscosity of these fluids increases with the addition of nanoparticles. Furthermore, the performance of these heat exchangers is influenced by the arrangement of baffles. Thus, in this paper, the study focuses on the heat transfer and entropy analysis of segmental, 25° and 50 helical baffles shell and tube heat exchangers. Heat transfer rate of the 25 helical baffles heat exchanger found to be the highest among the three heat exchangers studied in this research. Study indicates that shell and tube heat exchanger with 50° helical baffles exhibits lowest entropy generation among three different heat exchangers.     

Thermal resistance and capacity models for borehole heat exchangers

The detailed design and energy analysis of ground source heat pump systems requires the ability to predict the short‐term behavior of borehole heat exchangers (BHE). The application of fully discretized models leads to extensive computation times and a substantial effort in terms of pre‐processing work. On the contrary, analytical models offer simple, parameter input‐based modeling and short computation times, but they usually disregard the transient effects of heat and mass transport in the borehole and hence are not suitable for the prediction of the short‐time behavior. In order to combine the advantages of both types of models, the authors developed two‐dimensional thermal resistance and capacity models for different types of BHE. These models take the capacity of the grouting material with one capacity per tube into account and, therefore, the range of validity is extended to shorter times. The correct consideration of all thermal resistances between the fluid in the pipes, the grout capacities and the borehole wall is important because of the significant influence on the validity of the models. With the developed models, the modeling work and the computation time can be significantly reduced compared with fully discretized computations while precise results are still achieved. The validation of the suggested models against fully discretized FEM models shows a very good agreement. Copyright © 2010 John Wiley & Sons, Ltd.     

Thermal performance of aluminium‐foam CPU heat exchangers

This study investigates the performance of existing central processing unit (CPU) heat exchangers and compares it with aluminium‐foam heat exchangers in natural convection using an industrial set‐up. Kapton flexible heaters are used to replicate the heat produced by a computer's CPU. A number of thermocouples are connected between the heater and the heat sink being used to measure the component's temperature. The thermocouples are also connected to a data‐acquisition card to collect the data using LabVIEW program. The values obtained for traditional heat exchangers are compared to published data to validate experiments and set‐up. The validated set‐up was then utilized to test the aluminium‐foam heat exchangers and compare its performance to that of common heat sinks. It is found that thermal resistance is reduced more than 70% by employing aluminium‐foam CPU heat exchangers. The results demonstrate that this material provides an advantage on thermal dissipation under natural convection over most available technologies, as it considerably increases the surface‐area‐to‐volume ratio. Furthermore, the aluminium‐foam heat exchangers reduce the overall weight. Copyright © 2005 John wiley & Sons, Ltd.     

Thermal performance of inclined screen mesh heat pipes using silver nanofluids

This study presents the effect of silver nanofluid on thermal performance of inclined screen mesh heat pipe in cooling applications. Four cylindrical copper heat pipes containing two layers of screen mesh were fabricated and tested with distilled water and water based silver nanofluids with mass concentrations of 0.25%, 0.5% and 0.75% as working fluids. The experiments were performed at four inclination angles of 0°, 30°, 6° and 90°. The main focus of this study is to investigate inclined heat pipe performance with nanofluid. Experimental results indicate that the thermal performance of heat pipes was improved with nanofluids compared to water and thermal resistance of the heat pipes decreased with the increase of nanoparticle concentration. Moreover, the thermal performance of the heat pipes at inclination angle of 60° is found to be higher than other tested inclination angles, which shows the effect of gravity on heat pipe performance.     

Thermal performance of screen mesh wick heat pipes using water-based copper nanofluids

Fairly stable surfactant free copper–distilled water nanofluids are prepared using prolonged sonication and homogenization. Thermal conductivity of the prepared nanofluid displays a maximum enhancement of ～15% for 0.5 wt% of Cu loading in distilled water at 30 °C. The wall temperature distributions and the thermal resistances between the evaporator and the condenser sections of a commercial screen mesh wick heat pipe containing nanofluids are investigated for three different angular position of the heat pipe. The results are compared with those for the same heat pipe with water as the working fluid. The wall temperatures of the heat pipes decrease along the test section from the evaporator section to the condenser section and increase with input power. The average evaporator wall temperatures of the heat pipe with nanofluids are much lower than those of the heat pipe with distilled water. The thermal resistance of the heat pipe using both distilled water and nanofluids is high at low heat loads and reduces rapidly to a minimum value as the applied heat load is increased. The thermal resistance of the vertically mounted heat pipe with 0.5 wt% of Cu–distilled water nanofluid is reduced by ～27%. The observed enhanced thermal performance is explained in light of the deposited Cu layer on the screen mesh wick in the evaporator section of the heat pipe.     

Second-law-based performance evaluation of cooling towers and evaporative heat exchangers

In this paper, we present thermodynamic analysis of counter flow wet cooling towers and evaporative heat exchangers using both the first and second laws of thermodynamics. A parametric study is carried out to determine the variation of second-law efficiency as well as exergy destruction as a function of various input parameters such as inlet wet bulb temperature. Irreversible losses are determined by applying an exergy balance on each of the systems investigated. In this regard, an engineering equation solver (EES) program, with built-in functions for most thermodynamic and transport properties, is used. The concept of total exergy as the sum of thermomechanical and chemical parts is employed in calculating the flow exergies for air and water vapor mixtures. For the different input variables investigated, efficiencies were, almost always, seen to increase or decrease monotonically. We notice that an increase in the inlet wet bulb temperature invariably increases the second-law efficiency of all the heat exchangers. Also, it is shown that Bejan's definition of second-law efficiency is not limited in evaluating performance. Furthermore, it is understood that the variation in the dead state does not significantly affect the overall efficiency of the system.     

Numerical investigation on the use of nanofluids in natural convection heat exchangers using a mixture‐phase approach

In the present research, the behavior of a Newtonian nanofluid (water–Al₂O₃) in a mixture phase model approach is numerically examined. The process of heating is done in two different ways. Deterioration was found in the mean Nusselt number of a nanofluid in the mixture‐phase model approach when compared to the mean Nusselt number of pure water. However, in the single‐phase model there was an increase in the Nusselt number when compared to the Nusselt number of pure water. © 2011 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com/journal/htj ). DOI 10.1002/htj.20383     

Thermal performance of U-shaped serpentine microchannel heat sink using various metal oxide nanofluids

This study aims to evaluate the thermal performance and friction factor characteristics of the U-shaped serpentine microchannel heat sink using three different nanofluids. Two distinct nanoparticles, namely Al₂O₃ (alumina) and CuO (copper oxide), were used for the preparation of nanofluids using water and ethylene glycol (EG) as base fluids. Three nanofluids, namely nanofluid I (Al₂O₃ + water), nanofluid II (CuO + water), and nanofluid III (CuO + EG), have been prepared. The results showed that the thermal conductivity of nanofluids was increased for all concentrations (from 0.01 to 0.3%), compared with base fluids. The theoretical values derived from the relationship between the Darcy friction factor showed a clear understanding of the fully developed laminar flow. Thermal resistance for nanofluid III was lower than other nanofluids, resulting in a higher cooling efficiency. The nanofluid mechanism and the geometry of the U-shaped serpentine heat sink have led to the improvement in the thermal performance of electronic cooling systems.     

Wall-particles heat transfer in rotating heat exchangers

The wall-particles heat transfer coefficient has been measured in small-scale rotary drum heat exchangers. Experiments have been conducted with nine granular materials of different nature, with particle diameters ranging from 194 μm to 4mm. The effects of rotational speed (1–40 rev min⁻¹), filling degree (4–17%) and drum diameter (0.25 and 0.485 m) have been investigated. The experimental data have been correlated by a semi-empirical relationship, that includes a contact resistance at the wall, the heat capacity of the particles immediately adjacent to the wall and the heat penetration resistance of the bulk of the particle bed. The contact resistance is shown to be due to the roughness of the particles. A mean roughness height of 12 μm provides a good fit to the measured coefficients.