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Most of the absorbent-refrigerant pairings are evaluated considering single-effect vapour absorption refrigeration (VAR) cycle. The coefficient of performance (COP) based modelling of VAR enables its quick performance evaluation. The development of efficient and environmentally benign fluid pairing is required to replace conventional refrigerants. Operating parameters-based COP models are scarce for single-effect VAR systems, and fewer correlations are available for the COP estimation. The paper compares five performance-based models reported in the literature for operating absorption cooling systems with various refrigerant and absorbent pairing. The comprehensive mean absolute percentage error (MAPE) analysis was performed for five reported correlations for more than 1601 data points of different fluid pairings. Results revealed that RMSD and MAPE values seem significantly higher for the reported correlations apart from the recently developed COP correlation accounted for fluid parameters in prediction. This indicates that the earlier reported correlations only accounted for the specific fluid pairing and could not incorporate different fluid pairing, which has been considered a recently reported correlation that resulted in the significantly improved prediction ability for COP. The finding from this study highlighted that the newly registered COP prediction correlation could be beneficial for developing new single-effect VAR cycles as it accounted for both the operating parameters and fluid parameters.

This paper investigates the dynamic behavior of dual vapor compression Refrigeration (VCR) chillers, considering nine ecofriendly refrigerants such as R134a, R32, R1234yf, R1234ze(E), R717, R600, R...

The inception of the boiling, in a pool or flow boiling, is the formation of the vapor bubble at an active nucleation site that plays a crucial role in the boiling process and it becomes critical and unfolds many facets when channel size reduces to submicron. The detailed knowledge of the bubble dynamics is helpful in establishing the thermal and hydraulic flow behavior in the microchannel. In the current paper, bubble dynamics that include bubble nucleation at the nucleation site, its growth, departure, and motion along the flow in a microchannel(s) are discussed in detail. Different models developed for critical cavity radius favorable for bubble nucleation are compiled and observe that models exhibit large deviation. The bubble growth models are compiled and concluded that the development of a more generalize bubble growth model is necessary that would be capable of accounting for inertia controlled and thermal diffusion controlled regions. Bubbles at nucleation sites in a microchannel grow under the influence of various forces such as surface tension, inertia, shear, gravitational and evaporation momentum. Parametric analysis of these forces reckoned that the threshold between macro- to microchannel could be identify through critical analysis of such forces. Eventually, the possible impact of the various factors such as operating conditions, geometrical parameters, thermophysical properties of fluid on bubble dynamics in microchannel has been reported.

District Cooling Plants (DCP) is the sustainable way of energy consumption, refrigeration, and supply of chilled water to end-users via chillers and distribution networks. Thermodynamic assessment of the vapor absorption refrigeration (VAR) in the open literature has been performed for cooling capacity well below the actual requirement (less than 300 kW) of DCP. Therefore, this work aims to investigate energy and exergy analysis of large-scale single effect VAR cycles operated on NH3-water and water-LiBr systems for a cooling capacity of 4000 kW and assess several operating parameters’ influence on cycle performance. Different parameters such as evaporator operating pressure (water-LiBR = 0.010–0.020 atm, NH3-water = 5.39–6.20 atm), chilled water outlet temperature (288–297 K), absorber temperature (300–305 K), condenser temperature (310–315 K), cooling output (2900–4000 kW) and heat input (4300–5500 kW) to the generator are varied during the parametric analysis. The exergy destruction analysis of the cycle’s components is also included in the study. Overall, the COP (0.89) and Exergy efficiency (89%) of water-LiBr were found to be higher than that of an equivalent NH3-water system which is 0.697 and 81%, respectively. The maximum exergy destruction was found at the absorber unit (24%), followed by the generator (23%). Furthermore, the NH3-water and water-LiBr simulations were optimized for different parametric conditions, and optimal operating conditions were identified for large-capacity district cooling systems. Therefore, the findings will provide the roadmap for designing and optimizing large-scale DCP operated on VAR cycles.

Rapid urbanization resulted in the drastic expansion of the built infrastructure in urban areas. This eventually led to an increase in energy consumption in the residential and commercial sectors. An appropriate selection of the convective heat transfer coefficient correlation at the design stage is of vital importance as it directly affects the cooling load of the building and consequently buildings’ energy demand. In this context, the comparative analysis of existing convective heat transfer coefficient correlations (CHTCs) used in building simulation programs such as EnergyPlus, ESP-r, IES, IDA, and TAS, are assessed. These correlations are tested against the data of Liu et al.’s [1]. It is observed that some of CHTCs correlations show lower error and others exhibit a significant deviation. In the case of the CHTCs used EnergyPlus, it is observed that the TARP algorithm shows overall better prediction ability for windward, leeward, and roof surface. On the other hand, in the case of the ESP-r, different correlations show a good prediction ability for different surfaces. For windward surface: MoWiTT; for leeward surface: MoWiTT and McAdams; and for roof surface: Liu and Harris show closer prediction with an error of less than 30% among other correlations. The correlation used in IES, IDA, and TAS shows a large deviation for windward, leeward, and roof surfaces under considered input. Based on this analysis, it can be concluded that the choice of such CHTCs uses in the BES tool can lead to the significantly higher energy consumption of the building and hence need the expertise to make the appropriate selection.

Vapor absorption refrigeration (VAR) is a sustainable alternative to the conventional vapor compression refrigeration (VCR) cycle, owing to its lower non-renewable energy requirements and potentially for exploitation of renewable energy sources. Traditionally, the coefficient of performance (COP) of the conventional single effect VAR cycle is considerably lower than VCR cycles. This provides room for improvement which can be attained through double effect VAR cycles that provide relatively higher performance. The COP of the dual effect VAR cycle is enhanced due to the waste/rejected heat energy utilization from the condenser or the absorber into a secondary generator. Models that correlate the COP of the double effect VAR cycle with operating parameters are not available in the open literature, with Iyer’s correlation being the only exception. This work applies this COP correlation using literature data for double effect VAR that operate with a variety of refrigerant and absorbent pairs. A comprehensive Mean Absolute Percentage Error (MAPE) analysis is performed for more than 2028 data points of various fluid pairs. Results reveal that MAPE (86.6–839%) values appear to be quite high for the reported correlation. Furthermore, the model is optimized using the proposed data set, considerably reducing the MAPE up to 36.03%. The results also indicate that due to the lack of fluid-specific parameters, the application of this correlation may not support the development of new double effect VAR cycles. Therefore, it is crucial to establish a performance-based correlation that considers both operational parameters and fluid parameters to assess the performance of new and efficient dual effect VAR cycles.

Single- and two-phase heat transfers from a microchannel heat sink (MCHS) are emerging as innovative methods of cooling for diversified engineering applications including microelectronics, power pr...

Two phase flow initiates at Onset of Nucleate Boiling, where the first bubble emerges in the downstream flow direction. Bubble nucleation and growth in microchannel heat sinks is very conspicuous phase as the growing bubble can completely block the flow cross-section area at high heat flux. Hence, microchannels are more susceptible to flow boiling instability. In this paper effort has been put to study the bubble dynamics during bubble growth at nucleation site for microchannel in terms of non-dimensional energy ratio numbers and their variation from bubble inception until departure. New non-dimensional energy ratio is also proposed, which helps in differentiating inertia controlled and thermal diffusion controlled region during bubble growth at nucleation site. Further, possible impacts of these non-dimensional energy terms on critical heat flux are discussed.

Absorption refrigeration systems have been proposed as a reliable alternative to conventional vapor compression systems over the years. In the past, efforts have been made to comprehend the performance of absorption refrigeration systems based on thermodynamic analysis, experimentation and numerical simulations. However, absorption refrigeration systems are still not as efficient as vapor compression systems. Hence, the current review aims to highlight recent advances in the field of absorption refrigeration. Specifically, it covers recent development in the areas of falling-film, spray, bubble and microchannel absorbers. In the review, the roles of passive enhancement techniques including surface modifications, use of surfactants, and nanofluids used predominantly in falling film absorbers are discussed. Similarly, recent developments and findings in the study of bubble and spray absorbers are also presented in detail. Lastly, the review also covers recent developments in novel absorbers such as microchannel absorbers.

Inception of the boiling, in pool or flow boiling, is the formation of the vapour bubble at active nucleation site. The bubble dynamics plays an important role in the boiling process. It is critical as it unfolds many facets especially when channel size is reduced to submicron. The detailed knowledge of the bubble dynamics is helpful in establishing the thermal and hydraulic flow behaviour in microchannel. In this paper, the bubble dynamics which include bubble nucleation at nucleation site, its growth, departure and motion along the flow in a microchannel are discussed in details. Different models are developed for the critical cavity radius are compiled and observed that they show large variation when compare. The bubble growth models are compiled and concluded that a development of more generalized bubble growth model is necessary to account for the inertia controlled and thermal diffusion controlled regions. The bubble at the nucleation site in a microchannel grows under the influence of various forces such as surface tension, inertia, shear, gravitational and evaporation momentum. Parametric variations of these forces are critically studied and reckoned that the slope of these forces seems to be reduced beyond 500 μm. Eventually, possible impact of the various factors such as operating conditions, geometrical parameters, and thermophysical properties of fluid on bubble dynamics in microchannel has been reported.

Microchannel heat sinks (MCHS’s) are currently projected as twenty first century cooling solution. In the present numerical study, heat transfer enhancement in microchannel using extended surface has been carried out. Rectangular microchannel and cylindrical microfins are used in current study. Three different configurations of extended surface microchannel; Case I (upstream finned microchannel), Case II (downstream finned microchannel) and Case III (complete finned microchannel) are compared with plain rectangular microchannel. It is found that heat transfer performance of Case I is better than Case II. Case I even performs better than Case III at low Reynolds number. Average surface temperature is also significantly reduced in case of extended surface microchannels. Optimization of extended surface microchannel has also been successively carried out following univariate search method for number of fins, pitch, diameter and height of fins. Average heat transfer enhancement in optimized case is around 160% with acceptable pressure drop penalty.

Energy consumption and its associated consequences can be reduced by implementing district cooling strategies that supply low temperature water to a wide range of end users through chillers and distribution networks. Adequate understanding, performance prediction and further optimization of vapor compression chillers used widely in district cooling plants have been a subject of intense research through model-based approaches. In this context, we perform an extensive review of different modeling techniques used for predicting steady-state or dynamic performance of vapor compression liquid chillers. The explored modeling techniques include physical and empirical models. Different physical models used for vapor compression chillers, based on physics laws, are discussed in detail. Furthermore, empirical models (based on artificial neural networks, regression analysis) are elaborated along with their advantages and drawbacks. The physical models can depict both steady- and unsteady-state performance of the vapor compression chiller; however, their accuracy and physical realism can be enhanced by considering the geometrical arrangement of the condenser and evaporator and validating them for various ecofriendly refrigerants and large system size (i.e., cooling capacity). Apparently, empirical models are easy to develop but do not provide the necessary physical realism of the process of vapor compression chiller. It is further observed that DC plants/networks have been modeled from the point of view of optimization or integration but no efforts have been made to model the chillers with multiple VCR cycles. The development of such models will facilitate to optimize the DC plant and provide improved control strategies for effective and efficient operation.

One area of vital importance in the field of absorption refrigeration (ABR) is the search of alternative combinations of refrigerant/absorbent pairs that can enhance its performance relatively to the widely used H2O-NH3 or H2O-LiBr. In this work, three binary (H2O-LiBr, H2O-LiCl, H2O-LiI), three ternary (H2O-LiBr + LiI, H2O-LiBr + C2H6O2, H2O-LiBr + LiCl) and four quaternary (H2O-LiBr + LiCl + ZnCl2, H2O-LiBr + ZnCl2+CaBr2, H2O-LiBr + ZnBr2+LiCl, H2O-LiBr + LiI + C2H6O2) mixtures of refrigerant/absorbent are tested to investigate heat transfer and pressure drop characteristics across the solution heat exchanger used in ABR systems. Subsequently, seven different single effect absorption cooling flowsheet configurations are explored to assess the potential for exploitation of the generated results at the cycle level. The effects of the concentrations of the absorbent in the mixture of refrigerant/absorbent on heat transfer and pressure drop characteristics are addressed. As a result, H2O-LiI, H2O-LiBr + LiI and H2O-LiBr + ZnCl2+CaBr2 shows higher heat transfer coefficient and lower pressure drop among the tested binary, ternary and quaternary mixtures respectively. Furthermore, it is concluded that with increase in mass fraction of the absorbent in the mixture, the heat transfer coefficient is decreased. A process configuration with part of the refrigerant mixed with the strong solution through an ejector, prior to the solution heat exchanger, is found to exhibit highest coefficient of performance.

Due to rapid evolution in a wide range of technologies in twentieth century, heat dissipation requirement has increased very rapidly especially from compact systems. There is an urgent need for high-performance heat sinks to ensure the integrity and long life of these petite systems. Use of forced convection cooling has been limited by the requirement of the excessively high flow velocity and associated noise and vibration problems. Microchannel heat sink seems to be most reliable cooling technology due to its superior command over heat carrying capability. Understanding the flow boiling phenomena in microchannel heat sink experimentally and analytically has been topic of intense research in twenty first century. In this review paper, the experimental studies on flow visualization, pressure drop and heat transfer characteristics of microchannels presented by different researchers are summarized. Some different flow patterns observed in microchannel geometry such as bubble nucleation in thin film, periodic variation of flow pattern, flow circulation, bubble suppression and cross-channel flow are explained briefly. The influence of vapour quality, heat flux, mass flux and channel geometry on pressure drop and heat transfer characteristics of microchannel are reported. Different correlations reported for single and two phase heat transfer characteristics are compared. The comparative analysis showed that available single phase and two phase correlations are inconsistence and large variation is observed among these correlations for same channel geometry, fluid and operating condition. Different instabilities associated with microchannels are also briefly presented.

Critical heat flux (CHF) is a key design consideration for the systems involving heat dissipation through boiling application. It dictates the maximum limit of performance of heat transfer systems. Abrupt and substantial decrease in heat transfer coefficient is an indirect indication of occurrence of the CHF, which may cause complete burnout of heat transfer surface. Unlike conventional channels, CHF correlations for microchannels are limited and associated with significant variations. In the present paper, effort has been made to develop new CHF models applicable to a frequently occurring scenario of flow boiling in microchannels. The approach combines nondimensional analysis and an energy analysis based bubble growth model at an arbitrary nucleation site. Two separate CHF correlations for refrigerants and water have been developed following a semi-empirical approach. The proposed correlations show good agreement with available experimental data. The mean errors for the refrigerant and water cases are, respectively, found to be 21% and 27% for seven and six relevant datasets. Around 77% data of the refrigerant and 60% data of water are predicted within error band of ±30%. It is also found that influence of a certain energy ratio term (gravity to surface tension, denoted as πE4) is negligible for examined water CHF conditions.

Formation of the first bubble at nucleation site is an inception of the two phase flow in pool boiling and flow boiling. Bubble dynamics (bubble nucleation, growth, and departure) plays an important role in heat transfer and pressure drop characteristics during two phase flow in microchannels. In this paper, a simplified model has been developed for predicting bubble growth rate at nucleation cavity in microchannel. It is assumed that heat supplied at nucleation site is divided between the liquid phase and the vapor phase as per instantaneous void fraction value. The energy consumed by the vapor phase is utilized in bubble growth and overcoming resistive effects; surface tension, inertia, shear, gravity, and change in momentum due to evaporation. Proposed model shows a good agreement with available experimental works. In addition, the bubble waiting time phenomenon for flow boiling is also addressed using proposed model. Waiting time predicted by the model is also close to that obtained from experimental data.

Bubble dynamics i.e. bubble nucleation, growth and departure plays an important role in heat transfer and pressure drop characteristics during two phase flow of microchannels. A simplified mathematical model has been developed to predict the bubble growth rate in microchannels at nucleation cavity after its inception. It is assumed that heat supplied at nucleation site is divided between liquid phase and vapor phase as per instantaneous void fraction value. The energy consumed by vapor phase is utilized in overcoming evaporation, surface tension, inertia, shear and gravity effects. Proposed model shows good agreement (∼14 % error) with available experimental work. In addition, the physical phenomena of the bubble waiting time for flow boiling is also addressed utilizing proposed model. The waiting time predicted by the model is close to that obtained from experimental data.Copyright © 2013 by ASME