Comparison of different modeling approaches for minichannel evaporators under dehumidification

[EN] This paper firstly presents a comprehensive minichannel evaporator model (MCHX-1D-MB) based on fin theory coupled with the moving boundary technique along fin height. To validate the presented model, experimental data for R-134a and R-744 (CO2) minichannel evaporators were used. The proposed model successfully predicted the cooling capacity of R-134a and CO2 evaporators with mean absolute error values of +/- 1.8 and +/- 4.3%, respectively. Regarding the outlet air temperature, the mean absolute errors in the estimated results were +/- 0.43 and +/- 0.9 degrees C for R-134a and CO2 evaporators, respectively. Finally, to evaluate the impact of widely used assumption of cut fin on the air-side performance of minichannel evaporators, another model was developed (MCHX-1D-CF). The comparative study revealed that the most remarkable deviations between the two models appear when the evaporator operates under partially wet conditions, which were up to approximate to 12% in the latent heat transfer rate.The authors would like to gratefully acknowledge the financial support from the Spanish Ministry of Economy and Finance to project number ENE2014-53311-C2-1-P.Hassan, A.; Martinez-Ballester, S.; Gonzálvez-Maciá, J. (2019). Comparison of different modeling approaches for minichannel evaporators under dehumidification. Heat and Mass Transfer. 55(10):2901-2919. https://doi.org/10.1007/s00231-019-02622-0S290129195510Kim MH, Bullard CW (2001) Development of a microchannel evaporator model for a CO2 air-conditioning system. Energy 26:931–948Jin J, Chen J, Chen Z (2011) Development and validation of a microchannel evaporator model for a CO 2 air-conditioning system. 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Impact of Dehumidification Modelling on the Performance Prediction for Minichannel Evaporators

In this paper a two-dimensional numerical model for a minichannel evaporator is implemented. This model takes into account the variation of wall (fin and tube) temperature and moist air properties (temperature and humidity ratio) in both longitudinal and transverse directions. The validation of the current model is done with a well-defined analytical case, the results of both cases show a very good agreement and consistency. A case study from literature has been chosen to compare between the current model results, and the results from traditional ?-NTU method with adiabatic fin-tip assumption which used generally by many authors for modeling and analyzing the evaporator performance. The tube wall temperature is varied from 1.4 ? to 17 ?, under constant inlet air temperature and humidity ratio. These conditions allow different scenarios for the tube and fin (totally wet, totally dry, or partially wet). Deviations in results between the current model and the traditional ?-NTU approach are noticed, especially under partially wet fin condition. These deviations are mainly due to the assumptions which normally adopted by the ?-NTU method and fin theory such as; no variation in moist air temperature and humidity ratio along the direction between tubes, the whole segment (tube and fin) is usually assumed to be either completely dry or wet and no accounting for partially wet scenario

Visualization of the Refrigerant Flow at the Capillary Tube Inlet of a Houseold Refrigeration System

Capillary tube-suction line heat exchangers (CT-SLHX) introduce complex phenomena due to simultaneous 2-phase flow expansion and heat transfer such as: reverse heat transfer, flow hysteresis and flow oscillations. Some of the negative consequences of these phenomena are: noise due to re-condensation, which is becoming an important quality issue; and reduction of the SLHX effectiveness, which also affects the global efficiency. Studies about how to solve the noise problem show that it disappears when there is enough subcooling at the capillary tube inlet. This fact also supports the idea that two-phase flow at the capillary tube inlet contributes to re-condensation phenomenon in the CT-SLHX. Different reasons may explain it, e.g. the small compressor capacity compared the expansion device capacity of the capillary tube used. Another important consequence would be wasting condenser surface due to the two-phase outlet. Therefore, the main objective of this work is to assess experimentally the actual conditions taking place at the capillary tube inlet and find a solution to the problems mentioned above. An innovative test bench has been designed in order to visualize and analyze the phenomena occurring at the condenser outlet, along the filter and at the capillary tube inlet. This experimental bench test is connected to an A+++ no-frost household refrigerator equipped with a fin-and-tube evaporator, a tube and wire condenser and a variable-speed 7.24 cm³ hermetic reciprocating compressor. The refrigerant used is isobutane (R600a). In order to determine the refrigerant temperature, a set of thermocouples has been placed along the refrigerant loop, while a pressure transducer is installed at the condenser outlet. The mass flow rate is measured with a Coriolis meter installed at the compressor discharge line. The final part of the condenser and the filters were built with PFA (Perfluoroalkoxy) transparent pipes. Three different positions of identical filters were tested to analyze their influence on the flow configuration. The first filter is horizontally oriented, second and third ones are in a vertical position but with opposite flow directions. A system composed of three manual solenoid valves enables to test each of the three configurations independently from the two others. The transparent filters make possible the visualization of the refrigerant flow pattern at the capillary inlet. An adjustable system has been designed to modify the length of the capillary inside the filters. The set of experiments were tested in steady conditions, by using electrical heaters inside the cabinets to keep the setting point constant. Results show the description of the flow pattern at the capillary tube inlet and condenser outlet with the different capillary tubes and filter arrangements. The condenser outlet conditions were analyzed and the energy efficiency of the refrigerator was compared with a system with actual subcooled outlet conditions. Once characterized the refrigerant flow at the condenser outlet, a configuration has been proposed to ensure an effective subcooling and thus an improvement of the refrigerator performance

A Discussion about the Methodology to Validate the Correlations of Heat Transfer Coefficients and Pressure Drop during the Condensation in a Finned-Tube Heat Exchanger

As already demonstrated by others authors, when the performance of a heat exchanger is analyzed, a semi-empirical model allows getting good prediction of the experimental results provided that it is accompanied by the application of the suitable correlations for calculating heat transfer coefficients (HTC) and pressure drop (PD) in both refrigerant and air side. Many correlations for calculating these coefficients are available in literature, therefore choose the more suitable of them turns out to be not an easy task. This paper presents a discussion about the methodology to perform a comparison aimed to define the criterion for choosing the most suitable correlation to be used in a simulation model for prediction of heat exchangers performance in refrigeration systems. Differently from others works, where the results of each correlation are compared with those obtained by means specific experiments focused on the local phenomenon visualization, in this work the correlations are compared against the measurements of a complete finned-tube condenser integrated in a traditional air-to-water heat pump. The comparison has been carried out using a model able to discretize the heat exchanger through a finite volume method (FVM). The discussion is supported with a case study which was experimentally tested and numerically modeled. The condenser is a finned tube coil that characterized by two asymmetric circuits. The tube diameter is 7 mm and the fin pitch is 2.6 mm. The experimental test bench includes a single-speed 34.3 cm3 reciprocating-hermetic compressor, a plate evaporator and an electronic valve as expansion device. An accumulator allow adjusting the condenser sub-cooling. A specific test campaign has been designed in order to cover a wide range operating points of the condenser: different air velocities (from 1.5 m/s to 4 m/s), inlet temperatures (20-46°C) and sub-cooling (0-5-10°C) have been taken into account. All the correlations considered were programmed and simulated with the software IMST-ART. In order to avoid the overlapping of the effects of the different correlations, during the analysis of the two-phase flow, the heat transferred in the one-phase section of the exchanger and the air-side heat transfer coefficient were maintained constant. The analysis of air-side heat transfer coefficient and pressure drop was carried out in the same way but keeping constant the coefficients of the refrigerant side. Different accuracy parameters were defined in order to assist the final decision about which correlation is able to provide the best agreement with experimental data. The paper aims to present a discussion about the validation methodology, the analysis of the correlations impact on the model results rather than define the parameters to choose the best correlation

Global disparities in surgeons’ workloads, academic engagement and rest periods: the on-calL shIft fOr geNEral SurgeonS (LIONESS) study

: The workload of general surgeons is multifaceted, encompassing not only surgical procedures but also a myriad of other responsibilities. From April to May 2023, we conducted a CHERRIES-compliant internet-based survey analyzing clinical practice, academic engagement, and post-on-call rest. The questionnaire featured six sections with 35 questions. Statistical analysis used Chi-square tests, ANOVA, and logistic regression (SPSS® v. 28). The survey received a total of 1.046 responses (65.4%). Over 78.0% of responders came from Europe, 65.1% came from a general surgery unit; 92.8% of European and 87.5% of North American respondents were involved in research, compared to 71.7% in Africa. Europe led in publishing research studies (6.6 ± 8.6 yearly). Teaching involvement was high in North America (100%) and Africa (91.7%). Surgeons reported an average of 6.7 ± 4.9 on-call shifts per month, with European and North American surgeons experiencing 6.5 ± 4.9 and 7.8 ± 4.1 on-calls monthly, respectively. African surgeons had the highest on-call frequency (8.7 ± 6.1). Post-on-call, only 35.1% of respondents received a day off. Europeans were most likely (40%) to have a day off, while African surgeons were least likely (6.7%). On the adjusted multivariable analysis HDI (Human Development Index) (aOR 1.993) hospital capacity > 400 beds (aOR 2.423), working in a specialty surgery unit (aOR 2.087), and making the on-call in-house (aOR 5.446), significantly predicted the likelihood of having a day off after an on-call shift. Our study revealed critical insights into the disparities in workload, access to research, and professional opportunities for surgeons across different continents, underscored by the HDI

Impact of the Refrigerant Layout and Fin Cuts on the Performance of a Microchannel Condenser and a Gas Cooler

Microchannel heat exchangers (MCHX) play an important role in topics about charge reduction and transcritical cycles due to its high compactness and high mechanical strength. From a designer’s point of view, given an air-side area and face area, there are many options to design the refrigerant circuitry and aspect ratio of a MCHX. The paper presents numerical studies about the influence on the MCHX’s performance of these design parameters for a condenser working with propane and a gas cooler working with CO2. A technique to improve the MCHX effectiveness is to cut the fins along the middle section between neighboring tubes. Following same methodology as was used in previous study, the paper analyzes the improvement by cutting fins for different condensers layouts. A key point of the paper is that the parameters analyzed can only be assessed by models which take into account heat conduction between tubes, otherwise their effects would be hidden. Results illustrate the presence of an optimum circuitry in order to maximize heat transfer of condensers working in similar conditions. Improvement on the condenser heat transfer by cutting fins depends on the layout which turned to as much as 4%. Impact of heat conduction between tubes was higher and clearer in case of gas cooler

Dynamic Performance Simulation of a Household Refrigerator with a Quasi-Steady Approach

The paper presents a quasi-steady approach for modeling a household refrigerator. The model applies a novel methodology to perform the dynamic simulation. It uses a map of unit which contains high accuracy performance data of the refrigeration loop. This map is used by the model of the freezer and fresh-food air cabinets to determine the transient evolution of the air inside. This methodology allows obtaining high accuracy results, with high robustness and low computational cost. The validation of the model with experimental data is shown, by comparison of cabinet temperature and global power input. In order to understand some transient phenomena of the actual system operation, the model has been used to perform comparison studies between the operation of a real system and the equivalent quasi-steady system. Efficiency of both systems has been compared in order to detect energy losses sources. The energy losses analyzed are those related to controlling actions such as: compressor start-up and closing of the damper that supplies air flow to fresh-food cabinet. A discussion about their impact on performance and phenomena involved is presented