Experimental study on silica gel-LiCl composite desiccants for desiccant coated heat exchanger

Desiccant coated aluminum-fin is an important part in desiccant coated heat exchanger (DCHE). It will affect the overall performance of this novel solid desiccant component. In the paper, different silica gel-LiCl composite desiccant coated aluminum sheets were fabricated and some key parameters were studied. Nitrogen adsorption suggested that texture properties of composite desiccants were different from silica gel due to impregnated salt and combined action of silica sol and silica gel. The highest thermal conductivity of composite sheets was 5.8 Wm⁻¹K⁻¹, which is twice of silica gel coated one. Sorption kinetics indicated that composite sheets exhibited higher dynamic sorption quantities and faster sorption rates. Sorption isotherms were also obtained and fitted with Dubinin–Astakhov equation. Besides, dehumidification capacity of composite DCHE was tested and compared with silica gel one. Results indicated that enhanced dehumidification capacity can be achieved with use of composite desiccant in the DCHE system.     

A hybrid solar air conditioner: Experimental investigation

The removal of moisture from the supply air using conventional air conditioners (A/C) represents a considerable portion of the air conditioning load in hot and humid regions. Desiccant assisted A/Cs are used to address this issue. In this work, the performance of a hybrid A/C, which consists of a desiccant wheel, an enthalpy wheel, and a vapor compression cycle (VCC), is investigated experimentally. The effect of the process air stream's temperature and humidity, and the effect of the ventilation rate on the hybrid A/C performance are investigated. The experimental results show that the hybrid A/C is more effective than the standalone VCC in maintaining the indoor conditions within the comfort zone. The simulation of the complete hybrid solar A/C that uses a concentrating photovoltaic/thermal collector shows that a system coefficient of performance higher than unity is possible.     

Heat transfer analysis of fin-and-tube heat exchangers with flat and louvered fin geometries

This paper analyzes the fluid flow and heat exchange on the air side of a multi-row fin-and-tube heat exchanger. A comparison is given between fin-and-tube heat exchanger characteristics with flat and louvered fins in a wider range of operating conditions defined by Reynolds number (based on fin spacing and air frontal velocities). The detailed representation of calculated data for the louvered heat exchanger shows significantly better heat transfer characteristics and a slightly higher pressure drop. The CFD procedure was validated by comparing the numerical simulation results with the experimental results showing the minimal average Nusselt number deviation and an almost perfectly corresponding pressure drop.     

Evaluation of thermal contact conductance using a new experimental-numerical method in fin-tube heat exchangers

In a fin-tube heat exchanger the contact between fin collar and tube surface is obtained through mechanical expansion of tubes. Since the interfaces between the tubes and fins consist partially of metal-to-metal contact and partially of air, the features of heat transfer through the contact interfaces have not been fully investigated. The present study aims at the development of a new tool including an experiment and a numerical calculation for the estimation of the thermal contact conductance between the fin collar and tube surface, and pursues the evaluation of the factors affecting the thermal contact conductance in a fin-tube heat exchanger. Heat exchangers fabricated for the current study have been put to the test for heat balance in a vacuum chamber with water as an internal fluid. And a finite difference numerical scheme has been used for the data reduction of the experimental data to evaluate the thermal contact conductance. Fin-tube heat exchangers employed in the current research are of tube diameter of 7 mm with different tube expansion ratios, fin spacings, and fin types. The results of the present study imply that these parameters as well as hydrophilic fin coating have a significant effect on the thermal contact conductance. It has been discovered that the portion of the thermal contact resistance is not negligible compared with the total thermal resistance in a fin-tube heat exchanger, and this means that in order to reduce the thermal contact resistance thoughtful care should be taken in fabricating heat exchangers.     

Theoretical analysis of the thermal performance of a plate heat exchanger at various chevron angles using lithium bromide solution with nanofluid

Nanofluids technology has been rapidly developing over the last two decades. In this paper, the performance of a lithium bromide (LiBr) solution with and without nanoparticles in plate heat exchanger (PHE) for various chevron angles and mass flow rates was investigated. As a result, the heat transfer rate and the overall heat transfer coefficient in 60°/60° PHE is over 100% higher than that of 30°/30° PHE, and the effectiveness of the PHE in 60°/60° PHE is about 70% higher than that of 30°/30° PHE. By using nanoparticle in the working fluid, the heat transfer performance can increase significantly. The heat transfer rate of 3 vol.% nanofluids increased about 3–8% compare to that of LiBr solution for all chevron PHEs. Besides, the 60°/60° PHE using 3 vol.% nanofluids produced the largest heat transfer rate and heat exchange effectiveness under given operating conditions.     

Evaporation heat transfer and pressure drop of ammonia in a mixed configuration chevron plate heat exchanger

Ammonia is a naturally occurring environment friendly refrigerant with attractive thermo-physical properties. Experimental investigation of heat transfer and pressure drop during steady state evaporation of ammonia in a commercial plate heat exchanger has been carried out for an un-symmetric 30°/60° chevron plate configuration. Experiments were conducted for saturation temperatures ranging from −25 °C to −2 °C. The heat flux was varied between 21 kW m⁻² and 44 kW m⁻². Experimental results show significant effect of saturation temperature, heat flux and exit vapor quality on heat transfer coefficient and pressure drop. Current mixed plate configuration data are compared with previous studies on the same heat exchanger with symmetric plate configurations. This comparison highlighted importance of optimization in selection of the heat exchangers. Correlations for two phase Nusselt number and friction factor for each chevron plate configuration considered are developed. A Nusselt number correlation generalized for a range of chevron angles is also proposed.     

Thermal modeling of gas engine driven air to water heat pump systems in heating mode using genetic algorithm and Artificial Neural Network methods

The gas-engine driven air-to-water heat pump, type air conditioning system, is composed of two major thermodynamic cycles (including the vapor compression refrigeration cycle and the internal combustion gas engine cycle) as well as a refrigerant-water plate heat exchanger. The thermal modeling of gas engine driven air-to-water heat pump system with engine heat recovery heat exchangers was performed here for the heating mode of operation (in which it was required to model engine heat recovery heat exchanger). The modeling was performed using typical thermodynamic characteristics of system components, Artificial Neural Network and the multi-objective genetic algorithm optimization method. The comparison of modeling results with experimental ones showed average differences of 5.08%, 5.93%, 5.21%, 2.88% and 6.2% which shows acceptable agreement for operating pressure, gas engine fuel consumption, outlet water temperature, engine rotational speed, and system primary energy ratio.     

Impact of fin spacing on temperature distribution in adsorption cooling system for vehicle A/C applications

Effects of fin spacing on the temperature distribution in a finned tube adsorber bed are studied to decrease the temperature gradient inside the adsorber bed and minimize the adsorber bed to adsorbent mass ratio (AAMR) for vehicle air conditioning applications. Finned tube adsorber beds have shown higher specific cooling power and coefficient of performance, and low AAMR among the existing adsorber beds. A single-adsorber bed ACS with interchangeable heat exchangers is built and equipped with hermetic type T thermocouples. Two copper heat exchangers with 6.35 mm (1/4″) and 9.5 mm (3/8″) fin spacing are custom-built and packed with 2–4 mm silica gel beads. The experimental results show that by decreasing the fin spacing from 9.5 mm to 6.35 mm, the temperature difference between the fin and adsorbent reduces by 4.6 °C under the cycle time of 600 s and an adsorption to desorption time ratio (ADTR) of one. A greater reduction in the temperature gradient inside the adsorber bed with smaller fin spacing is observed for short cycle time operation, e.g. 600 s, compared to long cycle time operation, e.g. 1400 s. Finally, simultaneous comparison of the temperature gradient between the fins and AAMR against fin spacing indicates that the optimum fin spacing for a finned tube heat exchanger packed with 2–4 mm silica gel beads is about 6 mm.     

Thermo-hydraulic behavior of ice slurry in an offset strip-fin plate heat exchanger

Ice slurry is a promising alternative to conventional single-phase coolants in indirect refrigeration systems. In this paper, an experimental analysis of an offset strip-fin heat exchanger operating with ice slurry as working fluid is presented. The pressure drop and thermal performance have been determined. In order to obtain the partial thermal resistance in the ice slurry side an empirical correlation for the secondary fluid side was determined by applying the Wilson plot method in a set of tests performed previously. An empirical correlation in terms of the Colburn j-factor to describe the thermal behavior of the heat exchanger with ice slurry was obtained. On the other hand, the direct pressure drop measurements operating with different flow rates and ice fractions are shown and compared with values obtained with single-phase fluids. Pressure drop instabilities have been observed for flow rates lower than the nominal value provided by the manufacturer.     

Study on high efficient heat recovery cycle for solid-state cooling

Traditional vapor compression cycles (VCCs) use mainly halogenated refrigerants such as hydrochloroflurocarbons and hydrofluorocarbons that are considered as greenhouse gases. Therefore, their regulations are imposed on a global scale. As an alternative cooling technology other than VCCs, solid-state cooling technologies, such as magnetic cooling, thermoelastic cooling and electrocaloric cooling, demonstrate its advantage of not using greenhouse gases as working fluids. However, one of the most challenging issues of these solid-state cooling technologies is the relative high parasitic internal latent heat loss, which could significantly deteriorate the system performance. In order to improve the system performance of solid-state cooling, the authors propose a novel and high-efficient heat recovery (HR) cycle for solid-state materials with high thermal conductivity. The novel heat recovery process was first compared as an analog of spatial scale counter-flow heat transfer process. A simplified ideal model was developed to quantitatively investigate the heat recovery process performance limit and the physics behind the analogy of the spatial scale counter-flow heat transfer process. Experiment was conducted and 60% HR efficiency was achieved. In addition, a detailed dynamic model validated by the experiment results was developed and used to further investigate the limiting factors together with the theoretical analysis.     

Characterisation of the hydraulic maldistribution in a heat exchanger by local measurement of convective heat transfer coefficients using infrared thermography

A methodology was developed to characterise the heat exchangers' performance decrease due to two-phase flow maldistribution. It consists in measuring the spatial distribution of the local heat transfer coefficients with a rapid, non-invasive and fluid independent method. The method is based on the infrared (IR) thermography measurement of the temperature response to an oscillating heat flux. The amplitude of the measured temperatures is compared to the solution of an analytical model. The problem is solved iteratively to obtain the heat transfer coefficients. This method has been applied to evaluate the uneven phase distribution of an air–water mixture in a compact heat exchanger. The exchanger is composed of seven multiport flat tubes, a vertical downward header and horizontal channels. Experiments were performed for mass flux from 29 kg m⁻² s⁻¹ to 116 kg m⁻² s⁻¹ and for quality from 0.10 to 0.70.     

Simulation and validation of a hybrid-power gas engine heat pump

Hybrid-power gas engine heat pump (HPGHP) combines hybrid power technology with gas engine heat pump, which can keep the gas engine working in the economical zone. In this paper, a steady-state model of the HPGHP in heating condition has been established, the optimal torque curve control strategy is proposed to distribute power between the gas engine and battery pack. The main operating parameters of the HPGHP system are simulated on Matlab/Simulink and validated by experimental data, such as operating temperature, coefficient of performance (COP), fuel-consumed rate, etc. Heating capacity and COP of the heating pump system are validated under different ambient temperatures and water flow rates. The simulation and experiment results shows acceptable agreement, the maximum difference is respectively 8.9%, 5.9%, 9.5% and 8.2% for engine torque, motor torque, reclaimed heat and fuel-consumed rate. Based on the simulation results, HPGHP has the lowest fuel-consumed rate of 283 g (kWh)⁻¹ at engine speed of 3000 rpm; the PER of HPGHP system is about 15.9% and 11.4% higher than the GHP under the same load in Mode C and D.     

Defrosting performances of a multi-split air source heat pump with phase change thermal storage

Recently, multi-split air source heat pumps (ASHPs) have been used increasingly for space heating in cold winter. When it operates in frosting environment, periodic defrosting is necessary to maintain a high system performance. However, researches on its defrosting were few due to its high capital and complicated controls. To solve its fundamental problem of insufficient heat available during defrosting, a novel reverse-cycle defrosting (NRCD) method based on the phase change thermal storage has been developed. In this paper, comparative experiments using both standard reverse-cycle defrosting (SRCD) method and NRCD method were carried out on a multi-split ASHP unit with a phase change material heat exchanger (PCM-HE) acting as energy accumulator during heating operation and heat source during defrosting operation. Experimental results suggested that when using the NRCD method, the system performances, such as suction pressure and temperature, defrosting and heat-resumption durations, COP during defrosting operation can be effectively improved.     

Thermodynamic performance assessment of carbon dioxide blends with low-global warming potential (GWP) working fluids for a heat pump water heater

Blends of CO₂ with ten low-global warming potential (GWP) working fluids are evaluated for use in a heat pump water heater. The effects that the discharge pressure, component ratio, hot-water outlet temperature and chilled water inlet temperature have on the coefficient of performance (COP) of heat pump are analyzed when the pinch point of the heat exchange is considered. It is found that temperature glide of zeotropic mixture has a good thermal match with the temperature change of water as two pinch points appear in the gas cooler/condenser or evaporator. The good thermal match in the heat exchangers promotes the system COP. Addition of low-GWP working fluids to pure CO₂ can reduce the high-side pressure. The results show that CO₂/R41 and CO₂/R32 are suitable candidates for heat pump water heaters because of their high COP and low high-side pressure in comparison with those of a pure CO₂ cycle.     

An experimental investigation of refrigerant mixture R32/R290 as drop-in replacement for HFC410A in household air conditioners

In the present paper, the refrigerant mixture R32/R290 (68%/32% by weight) is investigated as the drop-in replacement for R410A in household air conditioners. The GWP of it is only 22% of that of R410A. Theoretical and experimental investigations are conducted on the performance of the air conditioners working with both R32/R290 and R410A. Experimental results show that the refrigerant charge amount of R32/R290 is reduced by 30.0%–35.0%; the cooling and heating capacities are increased by 14.0%–23.7%. For further reducing charge amount and flammability, the micro-channel heat exchanger (condenser) is employed to replace the finned tube one. Compared with the R32/R290 system using finned tube heat exchanger, the R32/R290 charge amount and the power consumption are reduced by 34.1% and 0.4%, respectively; the cooling capacity and the COP are increased by 6.4% and 6.8%, respectively.     

Experimental investigation of a reversible heat pump/organic Rankine cycle unit designed to be coupled with a passive house to get a Net Zero Energy Building

This paper presents an innovative reversible Heat Pump/Organic Rankine Cycle (HP/ORC) experimental unit designed to be coupled to a Net Zero Energy Building (connected to a 120 m² thermal solar roof and a ground heat exchanger). The system can operate in three different modes: an ORC mode to produce electricity when a large amount of heat is collected by the solar roof, a direct heating mode using exclusively the solar roof, and a HP mode for space heating during cold weather conditions. This paper describes a comprehensive experimental campaign carried out on a prototype unit using a modified HVAC scroll compressor (4 kWe). From the results, the technical feasibility of the system is demonstrated. A cycle efficiency of 4.2% is achieved in ORC mode (with condensation and evaporation temperature respectively of 25 °C and 88 °C) and a COP of 3.1 is obtained in HP mode (with condensation and evaporation temperature respectively of 61 °C and 21 °C).     

Reducing the power consumption of household refrigerators through the integration of latent heat storage elements in wire-and-tube condensers

This study evaluates the influence of latent heat storage elements on the condenser temperature of a commercial household refrigerator. In order to determine the power consumption and the temperature distribution, a standard wire-and-tube condenser is equipped with different heat storage elements (containing water, paraffin or copolymer compound). The results indicate that particularly the application of phase change materials (PCM) lowers the condenser temperature, which leads to a significantly reduced power consumption.     

Experimental study on the heat transfer and flow characteristics of nanorefrigerants inside a corrugated tube

The heat transfer and flow characteristics of MWCNT-R141b nanorefrigerant with different mass fractions have been studied through experiments. Experimental results were compared with existing correlations. A two-step method was used to prepare the nanorefrigerants. Span-80 was used as surfactant with an average particle diameter of 20 nm. Transmittance method was used to evaluate the stability of nanorefrigerants. Results showed that the stability of MWCNT-R141b nanorefrigerant, which is the added dispersant, was good during the experiments. The 0.3 wt% MWCNT-R141b nanorefrigerants had optimal heat transfer enhancement effects compared with pure refrigerants. The maximum Nusselt number increased by 40%. The specific pressure drop of nanorefrigerant increased as the Reynolds number (Re) increased, and the specific pressure drop of the pure refrigerant was minimum, which is similar to R141b.     

Experimental and numerical investigation of the effect of shock wave characteristics on the ejector performance

The entrainment performance and the shock wave structures in a three-dimensional ejector were investigated by Computational Fluid Dynamics (CFD) and Schlieren flow visualization. The ejector performance was evaluated based on the mass flow rates of the primary and secondary flows. The shock wave structures in the ejector mixing chamber were captured by the optical Schlieren measurements. The results show that the expansion waves in the shock train do not reach the mixing chamber wall when the ejector is working at the sub-critical mode. Decreasing of the shock wave wavelength increases the secondary mass flow rate. A three-dimensional CFD model with four turbulence models was then compared with the experimental data. The results show that the RNG k-ε model agrees best with measurements for predictions of both the mass flow rate and shock wave structures.