1,721,003 research outputs found
Void fraction and pressure gradient model for an adiabatic symmetric annular developed flow with entrainment
No full textThis paper presents a new method for the evaluation of the pressure gradient during annular two-phase flow in the presence of entrainment, based on the momentum balance on the liquid film trapped between the wall and the vapor core. The entrained liquid fraction, together with the atomization/deposition rates and shear stresses, are considered with the use of closure equations, in which the interfacial shear stress is calibrated by means of an experimental database. Through the resolution of a simple iterative procedure, the outcomes of the method are both pressure gradient and void fraction. The accuracy of the present model is tested with independent experimental databases from the literature, showing a very good agreement also with respect to existing available methods for pressure gradient and void fraction
Development and assessment of performance of artificial neural networks for prediction of frictional pressure gradients during two-phase flow
Full text linkThis paper presents a discussion on several possibilities to predict the frictional pressure gradient during two-phase flow, with both the application of artificial intelligence and the implementation of conventional correlations and predictive methods. To this purpose, a huge database of approximately 8000 data points has been collected from 49 sources available in scientific literature, including 23 working fluids and the following ranges of parameters: mass fluxes from 32.7 to 2000 kg/m2s, saturation temperatures from -190°C to +120°C (reduced pressures from 0.021 to 0.780), tube diameters from 0.5 to 14.0 mm.
This consolidated database has been used to train several artificial neural networks (ANNs), by using only two hidden layers (shallow neural networks) and evaluating the effect of: training and testing datasets choice (either test data included or outside the training domain), the number of neurons for each hidden layer (from 1 to 50), the type of output (either dimensional or non-dimensional), the type and number (from 1 to 22) of input parameters.
The best results (MAPE of 16.8% and 88% of data within ±30%) have been obtained by using the liquid-only two-phase multiplier
as non-dimensional output and 12 mixed input parameters. Compared to the statistics of well-established literature correlations for frictional pressure drop (best MAPE of 22% and 73% of data points predicted within a ±30% error range, provided by Mauro et al. mechanistic method), the ANN demonstrates therefore a higher general accuracy. However, the use of Artificial Neural Networks does not guarantee a physical trend, which is instead preserved with conventional prediction methods
Flow boiling of R452A: Heat transfer data, dry-out characteristics and a correlation
Full text linkThis paper presents an experimental investigation on two-phase heat transfer and dry-out occurrence for refrigerant R452A in a single horizontal circular stainless-steel tube having an internal diameter of 6.0 mm. The effects of mass flux (from 150 to 600 kg/m2s), saturation (bubble) temperature (from 23 to 55 °C) and heat flux (from 10 to 65 kW/m2) are investigated and discussed. Heat transfer coefficient and dry-out vapor quality data are then compared to R404A results in the same operating conditions, observing that the nucleate boiling contribution of the new blend is penalized by its very high temperature glide during evaporation. The assessment of some dry-out and flow boiling heat transfer coefficient prediction methods is finally carried-out and a correction factor on the nucleate boiling term is proposed to take into account the negative effect of the temperature glide difference on the mass diffusion in the liquid. By implementing this modification on two chosen asymptotic models, the statistical error analysis is considerably improved
Combined effects of refrigerant leakages and fouling on air-source heat pump performances in cooling mode
No full textFor electric heat pump systems (EHP), soft faults such as refrigerant leakages and heat exchangers fouling can lead to significant performance degradations, which can remain unidentified for a long period. This paper, through a digital model, wants to simulate the cooling season behavior of a case study air-source heat pump for residential air-conditioning, analyzing the performance degradation in case of both standalone and simultaneous occurrence of soft faults. The study is carried-out in different scenarios of not planned and ordinary maintenance of the machine and in typical climate conditions of Naples, Miami and Shanghai. The impact of faults on seasonal performances (SCOP) and total equivalent warming impact (TEWI) has also been analyzed. Results show that refrigerant leakages can have an impact on system performance higher than 25%, whereas heat exchangers fouling can downgrade the efficiency of approximately 15%. It is also found that, for each climate condition, a different maintenance strategy can halve the cases with SCOP penalization higher than 10%. Since faults have not the same probability of occurrence, a smart monitoring and a fault detection system can be the solution to guarantee good performances avoiding at the same time excessive maintenance costs
Flow boiling of carbon dioxide: Heat transfer for smooth and enhanced geometries and effect of oil. state of the art review
Full text linkThis paper presents a state-of-the-art review on flow boiling of carbon dioxide, including experimen- tal studies and correlations for smooth and enhanced tubes, with pure CO 2 and CO 2 /lubricant mixtures. Specifically, 5223 CO 2 heat transfer coefficient data in smooth tubes are collected, and the effect of the operating conditions is discussed. Additional 883 data points in microfin tubes and 1184 experimental heat transfer coefficients in smooth tubes with CO 2 /oil mixture are also collected, and the influence of the microfin structure and of the oil presence on the heat transfer mechanism is analyzed. The statistical analysis has highlighted that the CO 2 -based correlation of Fang et al. is very accurate ( MAE = 5.1%) for the smooth tube database, whereas the heat transfer coefficients in microfin tubes are satisfactorily predicted ( MAE = 30.5%) with the model of Mehendale. Among the available cor- relations for CO 2 /oil mixture in smooth tubes, the method of Gao et al. provides the highest accuracy ( MAE = 63.2%)
Heat transfer coefficient, pressure drop and dry-out vapor quality of R454C. Flow boiling experiments and assessment of methods
No full textFlow boiling heat transfer, pressure drop and dry-out vapor quality of low-GWP non-azeotropic mixture R454C are experimentally evaluated. All tests are carried out in a smooth, horizontal, stainless-steel tube having an internal diameter of 6.0 mm, heated by DC current by means of Joule effect. The influence of the operating parameters in terms of mass flux, heat flux and saturation pressure is investigated and discussed. Both mass velocity and heat flux have a positive effect on the flow boiling heat transfer coefficient, whereas the saturation pressure negatively affect the boiling performance. Frictional pressure gradients are instead seen to increase with increasing mass velocity and reducing saturation pressure, while the onset of dry-out is anticipated for higher heat and mass fluxes and is unaffected by the variation of the bubble temperature. The assessment of predictive methods is carried-out for experimental data not belonging to stratified flow and post-dry-out heat transfer, that could not be predicted by conventional flow pattern maps. Finally, some tested flow boiling heat transfer coefficient correlations provide better results when modifying their nucleate boiling contribution by taking into account the negative effect of the temperature glide during evaporation
Defrosting frequency optimization in a cooling system: Minimization of energy consumption vs reduction of the number of on/off cycles per hour. Definition of a methodology and assessment of commercial methods based on experiments
No full textThis paper presents the definition of a new method to obtain the optimum defrosting start time for a medium temperature refrigeration system with hot-gas bypass defrosting technique. A series of experiments is performed for a monoblock prototype placed in a cold room with controlled temperature (from −4 °C to +2 °C) and relative humidity (from 70% to 90%), whereas the ambient temperature is set to 25 °C and monitored in a calorimetric chamber. Specific tests are carried out by leaving the machine in operating mode up to the complete blockage of the evaporator, measuring the corresponding defrosting phase period and frost accumulated mass.
A new method for data reduction of experimental data is presented to calculate the average energy consumption of the whole cycle (on–off operation and defrosting). With these results from experiments, firstly, the average energy consumption is used as one of the decisional parameters together with the number of hourly on–off cycles of a multi-criteria optimization under constrained heating load versus cooling capacity ratio. Secondly, an assessment of the results achievable with commercial defrosting method is carried out. The new method for data reduction could be used to collect performance maps under lab conditions and then use it as multi-criteria decisional basis for a control system. The commercial methods monitoring air-evaporator temperature difference or pressure drop across the evaporator do not allow to keep the heating load versus cooling capacity below a desired threshold value under all operating conditions. Conversely, a control system based on a defined maximum allowable heating load versus cooling capacity ratio permits to control the temperature of the cabinet
Flow boiling of R32 in a horizontal stainless steel tube with 6.00 mm ID. Experiments, assessment of correlations and comparison with refrigerant R410A
No full textRefrigerant R32 is one option for the replacement of R410A in residential air conditioning and heat pump systems. This paper focus on the flow boiling characteristics of pure R32 in a single horizontal stainless steel tube with an internal diameter of 6.0 mm. A total of 437 experimental results are presented for twophase heat transfer coefficient and frictional pressure drop and the effect of the operating parameters in terms of mass flux (from 146 to 507 kg/m2 s), saturation temperature (from 24.6 to 40.6 °C) and imposed heat flux (from 2.4 to 41.2 kW/m2) is discussed. Several heat transfer coefficient and pressure drop data of R410A obtained in the same test section are also used for comparison purposes. Finally, predictions of the most quoted flow boiling heat transfer and frictional pressure drop methods are compared to experimental data to evaluate their agreement
Waste heat driven multi-ejector cooling systems: Optimization of design at partial load; seasonal performance and cost evaluation
Full text linkIn this paper, a seasonal performance analysis of a hybrid ejector cooling system is carried-out, by considering a multi-ejector pack as expansion device. A 20 kW ejector-based chiller was sized to obtain the optimal tradeoff between performance and investment costs. The seasonal performance of the proposed solution was then evaluated through a dynamic simulation able to obtain the performance of the designed chiller with variable ambient temperatures for three different reference climates. The optimized multi-ejector system required three or four ejectors for any reference climate and was able to enhance the system performance at partial load, with a significant increase (up to 107%) of the seasonal energy efficiency ratio. The proposed system was then compared to conventional cooling technologies supplied by electric energy (electrical chillers EHP) or low-grade heat sources (absorption chillers AHP) by considering the total costs for a lifetime of 20 years and electric energy-specific costs for domestic applications from 0.10 to 0.50 €/kWhel. The optimized multi-ejector cooling system presented a significant convenience with respect to both conventional technologies. For warmer climates and with high electricity costs, the minimum lifetime for the multi-ejector system to achieve the economic break-even point could be as low as 1.9 years
Experimental analysis on the hysteresis phenomenon during flow boiling heat transfer in a horizontal stainless-steel tube
No full textThis paper presents an experimental analysis on the hysteresis phenomenon occurring during flow boiling heat transfer in a horizontal stainless-steel tube having an internal diameter of 6.0 mm. Pure R134a is the working fluid employed, working at a saturation temperature and a vapor quality fixed to 40°C and 0.30, respectively, whereas two different mass velocities of 400 and 800 kg/m2s are studied. A dedicated experimental procedure has been carried-out to highlight the variation of the two-phase heat transfer performance due to the hysteresis effect. Specifically, tests performed at the same operating conditions but having a different thermal history (a higher imposed heat flux in the transient phase) were seen to provide lower wall superheat values and therefore higher heat transfer performances. The nucleate boiling contribution is isolated from the overall heat transfer and the parameters affecting the boiling curve behavior are highlighted and discussed, pointing out the differences when increasing and decreasing the imposed heat flux once a peak value is reached. The experimental trends are finally upheld by the boiling theory, considering the hysteresis effect as the activation of the surface cavities undergone a higher given thermal boundary condition
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