Pressure Drop Prediction Methods Research Articles

Experimental data for flow boiling of R450A in a horizontal tube

According to the new European policies aimed at the replacement of highly-pollutant greenhouse gases refrigerants, the scientific community has focused on new synthetic environmentally friendly substances to be employed in vapor compression cycles for the refrigeration and the air conditioning fields.On this regard, R450A is a new blend made up of R134a (42%) and R1234ze (58%), having a GWP equal to 604, and therefore represents an attractive solution as pure R134a substitute. In this paper, new flow boiling heat transfer and pressure drop data of R450A collected at the refrigeration lab of Federico II University of Naples are presented. The data refer to a horizontal stainless-steel tube having an internal diameter of 6.0 mm. The effects of mass flux (from 150 to 400 kg m−2s−1), heat flux (from 10 to 20 kW m−2) and saturation temperature (from 30 to 50 °C) are presented and discussed, together with the assessment of the most quoted two-phase heat transfer and pressure drop prediction methods.

Ammonia heat pumps for district heating: thermo-economic optimization analysis based on evaporator geometry for either direct expansion or chilled water configurations

Electric heat pumps are recognized as a key technology for decarbonization and have received increasing policy support in several countries over the last few years. These devices offer a highly efficient form of electric heating and could make an important contribution to the transition to a low carbon future, especially in case of district heating systems. Within this context, this paper presents a thermo-economic optimization analysis of a 7.35 MW electric heat pump system employed for district heating purposes and using ammonia as working fluid. The implemented code gathers multiple sub-models calibrated ad-hoc from real data or manufacturers’ datasheets. The heat exchangers are simulated by considering phenomenological equations and well-known heat transfer coefficient and pressure drop prediction methods. The optimization analysis is performed at constant condensation temperature that guarantees in/out hot water temperatures of 30/62 °C and is focused on the matching between compressor and evaporator heat exchanger, by considering either a direct expansion system or a chilled water heat pump. Both the coefficient of performance (COP) and the set-up costs are considered as optimization performance indicators for the construction of the Pareto front.

Experiment and Prediction of Pressure Drop in a Fiber–Powder Composite Material with Porous Structure for Energy Wheels and Air Cleaners

Energy wheels and air cleaners play crucial roles in building air conditioning systems. The former is essential for conserving energy in air conditioning systems, while the latter is necessary for ensuring the quality of indoor air. Pressure drop is a crucial parameter for both energy wheels and air cleaners, and it is essential to conduct theoretical and experimental investigations to aid in their design. In this study, we focused on the study of pressure drop in a fiber–powder composite material which can be used for both total heat exchange and air purification. Experimental tests were initially conducted to examine the impact of different parameters on the pressure drop in the material. Subsequently, based on the special fiber–powder structure of the material, two pressure drop prediction methods with different prediction strategies were proposed. The two prediction strategies were compared by analyzing the prediction accuracy of the two methods. As tested by experimental data, for both methods, the absolute prediction error was less than ±6 Pa when the pressure drop was below 50 Pa, and the relative prediction error was less than ±8% for most data sets when the pressure drop was greater than 50 Pa. Moreover, the root mean square error (RMSE) and mean absolute percentage error (MAPE) values of prediction for both methods were less than 4 Pa and 7% respectively. The test results show that although the prediction strategies are different, both prediction methods can obtain acceptable prediction results, and both methods are practical. This study is intended to serve as a valuable reference for the design of energy wheels and air cleaners.

Experimental analysis of high temperature flow boiling of zeotropic mixture R134a/R245fa in a plate heat exchanger

The potential to improve the performance of organic Rankine cycle systems by using zeotropic mixtures as working fluids has been demonstrated in the open literature. However, there are very few research works in the open literature studying the mixture flow boiling in plate heat exchangers, and none of the existing works address high temperature boiling, which is the prevailing working condition in the evaporator of organic Rankine cycle systems. This paper presents an experimental study of high temperature flow boiling of zeotropic mixtures of R134a/R245fa in a plate heat exchanger. Five compositions were tested at the reduced pressures 0.45, 0.55 and 0.65 and various heat fluxes and mass fluxes. In addition, the predictive performance of several existing prediction methods for the heat transfer coefficient and pressure drop were evaluated, and based on the results, new heat transfer and pressure drop prediction methods were developed. The experimental results suggest that the heat transfer is dominated by the nucleate boiling. The mixture having a 0.427/0.573 mass composition presents the largest heat transfer degradation, up to 42 %, compared with the pure fluids. As far as the pressure drop is concerned, the results suggest that the zeotropic mixtures have the same pressure drop characteristics as the pure fluids, and the five mixtures have almost the same frictional pressure drop due to their similar thermo-physical properties. Existing methods provide a good prediction of the heat transfer data. The new heat transfer correlations enable an even better predictive performance than those available in the literature, however, it remains to evaluate the predictive performance of the new correlation for other working fluids, plate heat exchanger geometries and working conditions than those considered in the paper.

Experimental thermal and hydraulic characterization of R448A and comparison with R404A during flow boiling

Refrigerant R404A is going to be banned in Europe for different applications due to its very high GWP of 3943. R448A is a suitable alternative (GWP of 1390) among other new mixtures, and is thus receiving considerable attention in literature. However, despite several overall system studies, there is a lack of flow boiling data for this new blend. This paper therefore presents an experimental investigation on two-phase heat transfer and pressure drop of R448A in a single horizontal stainless steel (AISI 316) tube with an internal diameter of 6.0 mm. The effect of the operating parameters is firstly analyzed for the whole range of vapor qualities, by changing the mass flux from 146 to 601 kg/m2 s and the bubble saturation temperature from 23.3 to 56 °C (reduced pressures from 0.266 to 0.554). For the heat transfer coefficient measurements, the imposed heat flux is also varied from 2.5 to 40 kW/m2, leading to a total amount of 460 data points. Several pressure drop and heat transfer coefficient data are then compared to those of conventional refrigerant mixture R404A at the same operating conditions, showing different relative importance of nucleate and convective boiling contributions. Finally, the agreement between the R448A experimental database and some two-phase boiling heat transfer coefficient and frictional pressure drop prediction methods is evaluated with a statistical analysis. The best results are respectively obtained with the correlations of Gungor and Winterton and of Friedel.

Experimental study and dynamic simulation of the continuous two-phase instable boiling in multiple parallel microchannels

Continuous vapor liquid two-phase flow means that both subcooled single phase and saturated flow boiling always exist in microchannels with the moving interface between them. The instable boiling phenomena and mechanisms are very complicated and needed to be deeply investigated and well understood. In the present study, both experiments and dynamics simulation of the continuous two-phase instable boiling in multiple parallel microchannels were systematically conducted. In the experimental aspect, characteristics of the unstable flow boiling in the microchannels were experimentally investigated using acetone as the working fluid and flow boiling processes were visualized with a high speed video camera simultaneously. The test section includes 16 parallel microchannels having a hydraulic diameter of 104.3 µm. The test mass flux ranged from 437.2 to 868.1 kg/m2 s and the test heat flux ranged from 0 to 100 W/cm2. The measured flow boiling heat transfer coefficients and two-phase frictional pressure drops are analyzed according to the observed flow phenomena and mechanisms. They were further compared to the existing flow boiling heat transfer and two-phase pressure drop prediction methods. The top three flow boiling heat transfer methods predict more than 65% of the heat transfer coefficients within ±30% while the top two pressure drop methods only predicted 43% of the measured data within ±30%. It shows that the continuous two-phase instable boiling mainly occurs at the mass flux larger than 607.6 kg/m2 s and the heat flux larger than 30 W/cm2. The backflow causes instability of the liquid and the two-phase interface which change the thermal resistances of the fluid. In the theoretical aspect, a dynamic simulation model was developed by using the thermal network method and was used to simulate the dynamic process of the instable boiling process. The simulated results were compared to the experimental data in this study and from the literature and the relative errors are within ±24.4%. According to the simulation and analysis, the mechanisms are that the axial thermal conduction in the channel walls generates a negative differential zone in the heat power characteristic curves, where the wall temperature decreases with increasing the heat flux in the negative differential zone. In order to suppress the oscillations in the boiling process, it is essential to eliminate the negative differential zone. Enhancing single phase forced convection heat transfer by 100% or enhancing flow boiling heat transfer by 50% can suppress the continuous two-phase instable boiling in the microchannels. Elongating the microchannel length and enhancing the axis thermal conduction can reduce the amplitudes of the oscillations by 60–80% at the conditions of G = 700 kg/m2 s, qeff = 70 W/cm2 and Tin = 30 °C.

A Study on Natural Circulation Flow Rate for Various Shaped Channels

The severe accident in Fukushima daiichi nuclear power plant revealed the importance of molten core cooling system without the electric power, then cooling system by natural circulation flow has attracted attention. The heat removal performance of this system depends on the natural circulation flow rate, so it is essential to predict the flow rate accurately for the safety design. The purpose of this study is to develop the evaluation method for the natural circulation, and to examine its accuracy dependence on the channel shape. Experiments were carried out at atmospheric pressure, using room temperature air-water flow. We used seven kinds of channels with various shape and diameter, and measured natural circulation flow rate and pressure drop for each channel. Predictive analysis was conducted by our method based on balance between driving force and pressure drop in the loop channel. The major results are as follows: (1) It is possible to predict the natural circulation flow rate with an error of less than 20 percent for various shaped channels. The average and standard deviation for the ratio between calculated and experimental flow rate is 0.98 and 0.047. (2) Two phase prediction method for pressure drop has maximum 15 % error. To improve the prediction accuracy, it is important to improve the prediction of the void fraction and the two-phase multiplier.

PWR蒸気発生器の二次系圧力損失の簡易的な予測手法に関する研究（U字管部の評価モデル構築検討）

PWR蒸気発生器の二次系圧力損失の簡易的な予測手法に関する研究（U字管部の評価モデル構築検討）

Pressure drop prediction in annular two-phase flow in macroscale tubes and channels

A new prediction method for the frictional pressure drop in annular two-phase flow is presented. This new prediction method focuses on the aerodynamic interaction between the liquid film and the gas core in annular flows, and explicitly takes into account the asymmetric liquid film distribution in the tube cross section induced by the action of gravity in horizontal tubes operated at low mass fluxes. The underlying experimental database contains 6291 data points from the literature with 13 fluid combinations (both single-component saturated fluids such as water, carbon dioxide and refrigerants R12, R22, R134a, R245fa, R410a, R1234ze, and two-component fluids such as water-argon, water-nitrogen, alcohol-argon, water plus alcohol-argon and water-air), vertical and horizontal tubes and annuli with diameters from 3mm to 25mm, and both adiabatic and evaporating flow conditions. The new prediction method is very simple to implement and use, is physically based and outperforms existing pressure drop correlations (mean absolute error of 12.9%, and 7 points out of 10 captured to within ±15%).

Two-phase pressure drop prediction in helically coiled steam generators for nuclear power applications

This study considers the prediction of the pressure gradient with water–steam two-phase flows through helically coiled steam generator tubes, focusing in particular on the operating conditions of low-medium pressure, low mass flux and low heat flux typical of once-through steam generators with in-tube boiling adopted in small modular nuclear reactor systems. Twenty-five widely used empirical correlations have been tested against an experimental pressure drop databank drawn together in this study containing 980 data points. Since no existing correlation is capable of collapsing and satisfactorily fitting the collected databank, a new pressure drop prediction method for helically coiled tubes is proposed. This new prediction method is very simple to implement, as it is based on the homogeneous flow model, is asymptotically consistent with straight tube two-phase flows and is largely superior in accuracy to existing prediction methods (mean absolute error of 7.3%, and 9 points out of 10 captured to within ±15%). The new prediction method is applicable for operating pressures in the range of 0.75–9.0MPa, mass fluxes from 400kg/m2s to 1191kg/m2s, heat fluxes up to 750kW/m2, tube diameters within 5–20mm and coil to tube diameter ratio above 32.4. Curvature effects on the pressure gradient in helical coil two-phase flows can be significant, particularly with high velocity flows in tight curvature coils where the centrifugal force is intense.

Modelling of a cross flow evaporator for CSP application: Analysis of the use of different two phase heat transfer and pressure drop correlations

Heat exchangers consisting of bundles of horizontal plain tubes with boiling on the shell side are widely used in industrial and energy systems applications. A recent particular specific interest for the use of this special heat exchanger is in connection with Concentrated Solar Power (CSP) applications. Heat transfer and pressure drop prediction methods are an important tool for design and modelling of diabatic, two-phase, shell-side flow over a horizontal plain tubes bundle for a vertical up-flow evaporator. With the objective of developing a model for a specific type of cross flow evaporator for a coil type steam generator specifically designed for solar applications, this paper analyzes the use of several heat transfer, void fraction and pressure drop correlations for the modelling the operation of such a type of steam generator.The paper after a brief review of the literature about the available correlations for the definition of two-phase flow heat transfer, void fraction and pressure drop in connection with the operation of steam generators, focuses attention on a comparison of the results obtained using several different models resulting by different combination of correlations. The influence on the analysis of the performance of the evaporator, their impact on significant design variables and the effective lifetime of critical components in different operating conditions, simulating the daily start-up procedures of the steam generator is evaluated. The importance of a good calibration of the model based on the comparison with some experimental data is recognised.

Heat transfer and flow characteristics during the formation of TBAB hydrate slurry in a coil heat exchanger

The objective of this paper is to investigate the pressure drop and heat transfer characteristics of a coil heat exchanger in which TBAB hydrate slurry is being formed from a 36.5 wt % TBAB solution. The coil heat exchanger has a 5.6 mm internal diameter. First tests were carried out with water in order to validate the experimental method. TBAB hydrates are formed by cooling the mixture of TBAB and water at temperatures from 20 °C to 11 °C. Experimental heat transfer coefficients and pressure drop of TBAB hydrate slurry have been obtained with solid concentrations in the range of 10–45 wt %. The results are compared with values for TBAB solution and water. Existing pressure drop prediction methods can reasonably predict the experimental data. The experimental data cannot be predicted with existing heat transfer prediction methods. A method is proposed for the prediction of heat transfer under hydrate formation conditions.

Pressure Drop of Two-Phase Flow Boiling with R-22 and R-290 in 7.6 mm Diameter Tube

The characteristics of two-phase flow boiling of R-290 are required for replacing R-22 that has been phased-out. The present study focuses on experimental pressure drop for R-22 and R-290. The experiment was run with heat flux of 5.09 kW/m2 to 19.03 kW/m2, mass flux of 114.91 kg/m2s to 751.74 kg/m2s and saturation temperature of 4.77°C to 18.12°C. The present result showed that pressure drop was affected by heat flux, mass flux and saturation temperature. Lower mass flux, heat flux and saturation temperature results in lower pressure drop. The pressure drop of R-290 is lower than that of R-22. Among the existing pressure drop prediction methods, Lokhart-Martinelli (1949) gives the best prediction for the present pressure drop data.

High resolution infrared measurements of single-phase flow of R245fa and R236fa within a compact plate heat exchanger, Part 1: Experimental setup and pressure drop results

This two-part paper presents experimental work to characterize thermal and hydraulic performances of a compact plate heat exchanger. Upward single-phase heat transfer and pressure drop of low pressure, liquid refrigerants R245fa and R236fa was investigated within a plate prototype fabricated with a 1 mm pressing depth and a chevron angle of 65°. Specifically in Part 1, experiments were carried out for Reynolds numbers ranging from 34 to 1615, corresponding to a range of mass flux from 8 to 384 kg m−2 s−1 and Prandtl numbers ranging from 4.9 to 6.5. The Fanning friction factor was found to be strongly dependent on the Reynolds number and the experimental pressure drop database was validated against several prediction methods available in the open literature. Fine resolution infrared measurements were performed to obtain local (pixel-by-pixel) heat transfer coefficient, fully described in Part 2, while the technique developed to reduce the experimental data is detailed in Part 1. Finally, a frictional pressure drop prediction method, which captured the entire experimental databank within a bandwidth of ± 20% and mean absolute error of 8.2%, was also provided.

Frictional Pressure Drop during Two-Phase Flow of Pure Fluids and Mixtures in Small Diameter Channels

Abstract Two-phase flow is widely encountered in minichannels heat exchangers such as air-cooled condensers and evaporators for automotive, compact devices for electronic cooling and aluminum condenser for air-conditioning applications. In the present work, frictional pressure drop during adiabatic liquid-vapor flow is experimentally investigated inside a single 0.96 mm diameter minichannel. Tests have been run with three mixtures of R32/R1234ze(E) (23/77%, 50/50% and 75/25% by mass composition) at mass flux ranging between 200 and 600 kg m−2 s−1. Since pressure drop has a strong influence on the two-phase heat transfer, it is crucial to have reliable pressure drop prediction methods for two-phase heat transfer modeling and optimization. Therefore, with the aim of extending its validity range, a model to calculate the frictional pressure gradient during two-phase flow in small diameter channels is tested against the present two-phase pressure drop database. An assessment is also done with two low-GWP refrigerants: the halogenated olefin R1234ze(E) and the hydrocarbon R290. The present model accounts for the effect of internal surface roughness as a function of the liquid-only Reynolds number.

Micro-orifice single-phase liquid flow: Pressure drop measurements and prediction

The dimensionless pressure drop was measured for six micro-orifices with diameters of 150μm, 300μm and 600μm, and thickness ratios between 1.87 and 6.93. The experiments were carried out with water for orifice Reynolds number between 6000 and 25,000, thus extending the range covered in previous researches in turbulent flow conditions. The dimensionless pressure drop was found to be a weak decreasing function of the Reynolds number, and was found to be unaffected by the micro-orifice diameter ratio or thickness ratio. The static pressure profiles measured immediately downstream of the micro-orifices were flat, indicating that the vena contracta is located within the micro-orifice. A new prediction method for the dimensionless pressure drop in micro-orifices at high Reynolds number was developed using available data.

Discussion on the validity of prediction tools for two-phase flow pressure drops from experimental data obtained at high saturation temperatures

In order to evaluate the validity of prediction tools for two-phase flow pressure drops for conditions of high saturation temperatures, this paper focuses on the comparison between new experimental results and theoretical results predicted with the commonly used methods. The original dataset was obtained in a horizontal 3.00 mm inner diameter during adiabatic flow with R-245fa as working fluid. The mass velocity ranges from 100 to 1500 kg m−2 s−1, the saturation temperature varies from 60 to 120 °C and the inlet vapor quality from 0 to 1. The database is composed of 249 data points covering four flow patterns: (i) intermittent flow, (ii) annular flow, (iii) dryout flow, and (iv) mist flow regimes. The dataset is compared against 23 well-known two-phase frictional pressure drop prediction methods. The effect of the saturation temperature and of the flow pattern on the ability of the methods to predict the frictional pressure drop was pointed out.

Pressure drop data and prediction method for enhanced external boiling tube bundles with R-134a and R-236fa

The pressure drops of external flow over enhanced tube bundles were experimentally obtained at both adiabatic and diabatic conditions using R-134a and R-236fa as test fluids. The tests were carried out at saturation temperatures of 5 and 15 °C, mass fluxes from 4 to 40 kg m−2 s−1, heat fluxes from 15 to 70 kW m−2 and inlet vapour qualities ranging from 10% to 90%. The frictional pressure drop was found to be primarily a function of mass flux and vapour quality. After comparisons were made with prediction methods in literature a new pressure drop prediction method was proposed for adiabatic and diabatic conditions. The proposed method is based on local measurements (4 and 8 tube rows) and flow conditions (evaluated per tube pitch) and the prediction method is well adapted to local incremental implementation for flooded evaporator design.

A mechanistic analysis of shell-side two-phase flow in an idealised in-line tube bundle

This paper reports on an experimental study of air–water mixtures flowing through an idealised shell and tube, in-line heat exchanger. Void fraction measurements are reported for the minimum gaps between the tubes at near atmospheric conditions. The pressure distributions around some tubes are also reported. These data are combined with data available in the open literature to investigate pressure drop and void fraction prediction methods for these heat exchangers. The data are shown to be flow pattern dependent. Criteria for flow pattern boundaries are deduced from previously published flow maps. Void fraction data in the maximum gap between the tubes are shown to be compatible with the drift flux model and to be different in magnitude to the minimum gap values, which are shown to result from acceleration phenomena in the gaps between the tubes. The pressure drop data are analysed through a one-dimensional model that incorporates separation and re-attachment phenomena. The frictional pressure drop is shown to depend on a liquid layer located on the upper portion of the tubes at low gas velocity and on acceleration effects at high gas velocity.

Flow regimes and two-phase pressure gradient in horizontal straight tubes: Experimental results for HFO-1234yf, R-134a and R-410A

Two-phase flow regime visualizations of HFO-1234yf and R-134a in a 6.70 mm inner diameter glass straight tube have been simultaneous investigated by top and side views with a high speed high resolution camera. No major difference was observed between both refrigerants. HFO-1234yf flow regimes were satisfactorily predicted by the Wojtan et al. [1] flow pattern map. In addition, 819 pressure drop data points measured during two-phase flow of refrigerants HFO-1234yf, R-134a and R-410A in horizontal straight tubes are presented. The tube diameter ( D) varies from 7.90 to 10.85 mm. The mass velocity ranges from 187 to 1702 kg m −2 s −1 and the saturation temperatures from 4.8 °C to 20.7 °C. The results are compared against 10 well-known two-phase frictional pressure drop prediction methods. For the entire database, the best accuracy is given by the method of Müller-Steinhagen and Heck [2] with around 90% of the data predicted within a ±30% error band. An analysis was carried out on the maximum pressure gradient and on the corresponding vapor quality. A statistical analysis for each flow regime was also carried out.