International Journal of Air-Conditioning and Refrigeration

Heat transfer coefficient as an important characteristic in heat exchanger design is determined by the correlation developed from previous experimental work or accumulation of published data. Although discrepancies still exist between the existing correlations and practical data, several researchers claimed theirs as a generalized heat transfer correlation. Through optimization method, this study predicts the heat transfer coefficient of two-phase flow of propane in a small channel at the saturation temperature of 10${}^{\circ }$C using two categories of correlation — superposition and asymptotic. Both methods consist of the contribution of nucleate boiling and forced convective heat transfer, the mechanisms that contribute to the total two-phase heat transfer coefficient, which become as two objective functions to be maximized. The optimization of experimental parameters of heat flux, mass flux, channel diameter and vapor quality is done by using genetic algorithm within a range of 5–20kW/m², 100–250kg/m²s, 1.5–3mm and 0.009–0.99, respectively. In the result, the selected correlations under optimized condition agreed on the dominant mechanism at low and high vapor qualities are caused by the nucleate boiling and forced convective heat transfer, respectively. The optimization work served as an alternative approach in identifying optimized parameters from different correlations to achieve high heat transfer coefficient by giving a fast prediction of parameter range, particularly for the investigation of any new refrigerant. In parallel with some experimental works, a quick prediction is possible to reduce time and cost. From the four selected generalized correlations, Bertsch et al. show the closer trend with the reference experimental work until vapor quality of 0.6.

Presently, to enhance the thermal efficiency of a gas turbine power plant, turbine inlet air cooling (TIAC) is the widely used technique. The conventional refrigeration methods like vapor compression refrigeration and evaporative cooling need electric power, hence absorption and adsorption refrigeration systems are attractive options as they can be powered using the waste heat energy of the exhaust gases. Adsorption system has advantages over absorption system like scalability, requirement of lower heat source temperature, absence of corrosion and crystallization. This paper focuses on the thermodynamic analysis of waste heat powered adsorption chiller used for the cooling of intake air to enhance the net power output of the gas turbine power plant. This paper also presents a comparative analysis of the vapor-adsorption cycle-based TIAC system for four different refrigerants viz. HFC-134a, carbon dioxide, ethanol and ammonia with the motive of finding a substitute refrigerant for HFC-134a which has a high global warming potential (GWP). The adsorption chiller is mathematically modeled in MATLAB with activated carbon as the adsorbent and each one of carbon dioxide, ethanol and ammonia as the adsorbate. The variation of the coefficient of performance (COP) and specific cooling effect (SCE) with varying adsorption temperatures is presented for each pair. The net power output and primary energy rate (PER) improvement of the gas turbine power plant at different ambient temperatures are also discussed. It is observed that ammonia can improve the power plant performance significantly better compared to the other three refrigerants at ambient temperatures less than 40${}^{\circ }$C.

Energy and economic analyses of corrugated plate evaporator have been performed in this study by using brine-based hybrid nanofluids as secondary refrigerant for low temperature applications (milk chilling, ice making and fish freezing). Various hybrid nanofluids containing alumina with different nanoparticles (copper, silver and multi walled carbon nanotube) dispersed in ethylene glycol/water, propylene glycol/water, calcium chloride/water and potassium acetate/water brines have been considered. A comparison has been made based on overall heat transfer coefficient, evaporator size (heat transfer area), pump work, COP, annual cost and payback period. The maximum reduction in heat transfer area has been observed for PG brine-based alumina–silver hybrid nanofluid. Whereas, the maximum reduction in pump work and augmentation in COP have been obtained for PG brine-based alumina–MWCNT hybrid nanofluid. EG brine-based Alumina–Cu hybrid nanofluid yields minimum annual cost. Most of the nanofluids (except PG brine-based Al₂O₃–Ag nanofluid) seem to be not beneficial for practical use due to comparatively long payback period; however, that can be significantly reduced in future by decreasing nanoparticle cost and increasing nanofluid stability. The present study reveals that the brine-based hybrid nanofluids may be the potential option as next generation secondary refrigerants for low temperature applications.

Thermoelectric generator (TEG) is a promising option in waste heat recovery with various advantages including nonmoving components, very low operating noise and overall high stability. However, low power generation efficiency of TEGs is still concerned for application in wider fields. The objective of the present study is to investigate the parameters affecting the performance and efficiency of segmented TEG. The physical parameters including length ratio, length-to-area ratio and thickness of conducting plate were considered. In addition, the effect of varying hot side temperature was analyzed. Performance of segmented TEG was compared based on the power generation and conversion efficiency. The results of the investigation showed that increasing Bi₂Te₃ length in the segmented TEG made up of Bi₂Te₃ and SiGe increased the maximum power generation and efficiency. As length-to-area ratio increased, the power generation decreased, however, efficiency increased slightly. In addition, as the hot side temperature increased from 150${}^{\circ }$C to 650${}^{\circ }$C, the power generation and efficiency both increased. Power generation and efficiency of segmented TEG increased when the conducting plate thickness was increased. The power generation and efficiency were 39.2% and 51.9% higher for TEG with asymmetrical element than TEG with symmetrical element showing that asymmetrical element would be better option for same thermoelectric element volume.

R-404A condensation heat transfer and pressure drop data are provided for 7.0mm O.D. smooth and microfin tubes. Tests were conducted for a range of mass fluxes (from 80 to 200kg/m²s) and quality (from 0.2 to 0.8). The heat flux was 6kW/m² and saturation temperature was 45${}^{\circ }$C. It was found that both the heat transfer enhancement factor and the pressure drop penalty factor increase as mass flux increases. The range of pressure drop penalty factor (0.99–1.27) was smaller than that of heat transfer enhancement factor (1.21–1.96). Smooth tube heat transfer coefficients and pressure drops are reasonably predicted by Shah [An improved and extended general correlation for heat transfer during condensation in plain tubes, Int. J. HVAC&R Res. 15 (2009) 889–913] and Jung and Radermacher [Prediction of pressure drop during horizontal annular flow boiling of pure and mixed refrigerants, Int. J. Heat Mass Transfer 32 (1989) 2435–2446] correlation, respectively. For the microfin tube, however, all the existing correlations do not adequately predict the present data. Poor predictions may be attributed to the lack of R-404A and low mass flux data in their database.

Solar absorption cooling technology has enormous potential for air-conditioning applications as the need for cooling coincides mostly with the availability of solar radiation. Due to the geographical nature and vast territory of Algeria and the presence of many isolated rural areas, there is a great need for autonomous systems that use only solar energy without auxiliary boiler to meet the cooling demand in summer. In this study, we propose a dynamic simulation model for solar autonomous absorption air-conditioning systems developed using the TRNSYS-EES software. The model used to study the feasibility and evaluate the functioning of the system under Batna, Algeria weather conditions. The proposed system uses solar thermal flat plate collectors to drive a small capacity single-effect LiBr/water absorption chiller to meet the cooling demand of a typical low-energy residential building covering a total floor area of 120m². A model of 4.5kW commercial absorption chiller has been modeled with EES software. The validation results showed excellent agreement between the prediction and the experimental data ($R^2>0.988$) and proved the model ability to predict the performance of the chiller. The analysis of dynamic results indicates that the proposed system and selected size enable the chiller to work for a duration of up to 10h a day. This allowed ensuring a sufficient and continuous cold water supply (between 8${}^{\circ }$C and 18${}^{\circ }$C) and maintaining the comfort temperature between 26${}^{\circ }$C and 28${}^{\circ }$C.

This study experimentally investigated the flow pattern, void fraction, and evaporation heat transfer characteristic of R134a upward flow in a vertical narrow rectangular channel having a hydraulic diameter of 0.99mm, resembling a plate heat exchanger. Experiments were performed with mass velocities and vapor quality ranging between 30–200kgm${}^{-2}$s${}^{-1}$ and 0.05–0.9, respectively, at a saturation temperature of 15${}^{\circ }$C. Flow patterns were classified into bubble, slug, churn, and annular flows. The void fraction increased with increasing quality, and decreased with decreasing mass velocity in the low-quality region. Further, the influence of flow inlet/outlet positions remarkably appeared when the superficial gas velocity was low. The observed flow patterns and the measured void fraction were compared to that in previous studies. The effects of mass velocity and heat flux on the evaporation heat transfer coefficient were small, and the heat transfer through the thin liquid film was dominant.

The present study aims to experimentally investigate the condensation frosting characteristics of the SAM-coated superhydrophobic surfaces under various environmental conditions. The extensive experiments were carried out under the well-controlled conditions with the surface temperature ranging from $-4^{\circ }$C to $-12^{\circ }$C, the relative humidity of 60%, and the air temperature of 24${}^{\circ }$C. The dynamics of condensation frosting was analyzed from the captured images by using CMOS camera. From the results, it was found that even if the substrate remained at a subfreezing temperature, the spontaneous freezing of supercooled condensates did not occur because of the free energy barrier of ice embryo at the interface of the substrate-supercooled droplet. The onset of condensate freezing was also triggered by probabilistic ice nucleation. In particular, the delay of the onset time of freezing was found for the nanostructured superhydrophobic surface and substantially dependent on the coolant temperature. Moreover, it observed the presence of an inter-droplet ice bridging between a frozen droplet and neighboring supercooled droplets.

An experimental research was performed to study the effect of time lapse on the change of water-Al₂O₃ nanofluid thermal conductivity and its convection heat transfer. The size of Al₂O₃ nanoparticle size was 20nm and 70nm, and initial volumetric concentration range was from 0.5% to 3%. A surfactant was added to the nanofluid and the change of thermal conductivity and convection heat transfer was also measured. The surfactant was Sodium Dodecyl Benzene Sulfonate (SDBS) and its mass fractions in the nanofluid were from 0.5% to 3.0%. Thermal conductivity of water and nanofluid was measured by the transient hot wire method. The accuracy of the measurement method was confirmed by the measurement error with 0.92% for distilled water at 20${}^{\circ }$C. The thermal conductivity of the nanofluid without SDBS increased up to 11.3% and the enhancement decreased with time lapse. The reduction of thermal conductivity enhancement with the time lapse could be retarded by the addition of SDBS and its effect became higher with the increase of its mass fraction. The convection heat transfer characteristics of the nanofluid was measured in a small cooling system. Compared with pure water, nanofluid convection heat transfer could be enhanced but higher pressure drop also occurred. Compared with the convection heat transfer enhancement for the nanofluid without SDBS, the addition of SDBS decreased the enhancement at the initial stage of the experiment, but it could retard the reduction of convection heat transfer with time lapse.

The effects of oil wettability on the performance of a cyclone-type oil separator was studied through in situ experimentation and in a real refrigeration system. Based on previous research, the geometry of the oil separator in the present study was designed with an oil recovery device installed at its bottom to mount it on the actual refrigerator. The performance of the oil separator without surface treatment was predicted by applying design correlations proposed in the open literature, which were then compared with the experimental data. Through surface treatment, oleophilic or oleophobic properties were given to the inner wall and helix of the oil separator, and its performance was measured in a real refrigeration system. Oil wettability had a great effect on the performance of the oil separator, and in order to obtain high separation efficiency, oleophilic properties were found to be advantageous not only in the inner wall but also in the helix.