Frictional Pressure Drop Research Articles

Unveiling impacts of heat flux distribution on forced convection in horizontal evacuated tubes

Integration of evacuated tubes with solar thermal desalination can significantly enhance thermal efficiency and water production. To provide optimization strategies for such desalination systems, 3D numerical simulations were performed to analyze the influence of the heat flux distribution on forced convection in horizontal evacuated tubes in terms of pressure drop, secondary flow, temperature distribution, and Nu. The improved method considered multi-layer wall heat transfer, fluid–solid coupling heat transfer, and temperature dependent fluid properties. Non-uniform heat fluxes significantly affected the friction pressure drop in the laminar regime, but were less pronounced in the transition and turbulent regimes. At low Re, the backward non-uniform heat flux strengthened the secondary flow while the forward non-uniform heat flux suppressed it. The rapid warming of the wall temperature occurred over a short distance due to the difference in thermal conductivity between the fluid and solid domains. The non-uniformity of heat flux distribution moderately affected average fluid temperature. The influence of heat flux distribution on secondary flow and average temperature weakened as Re increased. Furthermore, a correction factor was introduced to quantify the impact of heat flux distribution on Nu. For forward and backward non-uniform heat fluxes, it assumes values of 0.94 and 0.86 respectively.

A comprehensive study with high-speed camera assisted visualizations of HFO-1234ze(E) condensation inside an enhanced tube

Flow condensation of HFO-1234ze(E) inside a helical micro-finned tube of 4.28 mm ID underwent experimentation. Flow patterns were visualized with a high-speed camera while varying vapor qualities from 0.01 to 0.95 and mass fluxes from 100 to 400 kg m−2 s1, at saturation temperature of 30 °C. Intermittent, stratified-wavy, transitional, and annular flow pattern types were identified. The flow pattern map of Doretti et al. (validated by Cavallini et al.) accurately predicted the flow patterns. Heat transfer coefficients (HTCs) and frictional pressure drops (FPDs) were also measured for mass fluxes ranging from 100 to 600 kg m−2 s−1 at saturation temperatures of 30 °C and 40 °C. Both HTC and FPD elevated with higher mass fluxes and vapor qualities which stemmed from the presence of observed shear stress-driven annular flow patterns. HTC and FPD are generally higher at lower saturation temperatures. Experimental HTC and FPD have been compared against values estimated with empirical models. Furthermore, the average enhancement factor of the micro-finned tube was 2.94 with an area enhancement factor of 2.39. Plotting enhancement factor versus non-dimensional gas velocity provided detailed insight into the augmentation effect of micro-finned tubes under different operating conditions.

Development and assessment of performance of artificial neural networks for prediction of frictional pressure gradients during two-phase flow

This 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 ΦLO2 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.

Study on Two-Phase Pressure Drop of Methane during Flow Boiling in Mini Channel

For LOX/LCH4 variable thrust rocket engine, the propellant methane is traditionally selected as the coolant in regenerative cooling channel (RCC). With the decrease of engine thrust, the mass flow rate of coolant methane decreases gradually. At low engine thrust, the coolant methane is usually in a subcritical state. The heat transfer deterioration of subcritical methane occurs in RCC, which may cause thrust chamber wall ablation. The two-phase pressure drop data of methane are crucial parameters for the design and optimization of RCC. But it is rarely to find such measured frictional pressure drop data of methane in open published literature. The two-phase pressure drop of methane during flow boiling in the single mini channels with the diameters of 2.0 mm are investigated systematically. Effects of the mass flux (582.19~1755.48 kg/m2·s), inlet pressure (0.56~3.55 MPa), heat flux (53.25~318.68 kW/m2) on the frictional pressure drop of methane are discussed. The results show that the frictional pressure drop of methane during flow boiling increases with mass flux and inlet pressure at the experimental conditions, and heat flux shows weak effect on the frictional pressure drop. The comparisons of the experimental data with the predicted value by existing six correlations are analyzed. Contrary to the conventional channels, homogeneous model yields better prediction than five separated flow models. Present experimental results can provide reference for the design and optimization of RCC in LOX/LCH4 rocket engine.

Numerical analysis of sodium-argon two-phase flow in narrow channels under severe accident conditions in pool type SFR

A two-phase flow model for sodium-argon mixture flow through the reactor vessel head penetrations under the energetic core disruptive accident in a sodium fast reactor (SFR) is presented. The model evaluates the total sodium mass in sodium-argon mixture released to the reactor containment building (RCB) through penetrations. The driving pressure at the penetration entrance is given as input. The two-phase frictional and local pressure drops, inertial and piezo-metric heads are accounted for in the model. Depending on the argon volume fraction, either a homogenous or separated flow approach is adopted to evaluate the mixture flow. Reactor-scale analysis show that the presence of argon in vessel head penetrations can significantly mitigate the total sodium release mass during the slug impact phase. Also, the two-phase frictional pressure drop plays a mitigating role and needs to be included in the model formulation to avoid overestimation of the sodium release mass. Fluid-structure interaction effects such as vessel head bending and hold-down bolt elongation have negligible influence on sodium release mass as long as the deformations are within the elastic limit. The model is conceived to be part of an integrated code system to evaluate the sodium fire and the maximum RCB pressure under severe accident conditions in a pool-type SFR.

Comprehensive study of synergistic effects of phase separation structures and electric fields for enhancing the heat transfer performance of minichannel heat sinks

In this paper, the effects of the phase separation structure on the enhanced boiling heat transfer mechanism and bubble dynamics were investigated. Two types of phase-separation structures and one type of conventional minichannel were manufactured by ultrafast laser technology. Flow boiling experiments of a glycerol-water solution were conducted under various heat flux and electric field conditions. Entropy generation analysis was employed to comprehensively analyze and assess energy losses and irreversibility within the system. Response surface analysis method was introduced to optimize operational and design parameters for achieving optimal experimental design. Moreover, the coupling effect of the phase-separation structure and electric field was observed to have a significant impact on the enhancement of heat transfer. The visualization results indicate that, compared to conventional parallel minichannels, the countercurrent minichannel with phase-separation structure leads to a gradual decrease in the aspect ratio of confined bubbles downstream (in contrast to the increasing aspect ratio of confined bubbles downstream in general minichannels), and smaller bubbles at the upstream of adjacent minichannels gradually disappear and diminish due to the condensation effect. The experimental results demonstrate that the porous phase-separation structure plays a significant role in enhancing heat transfer performance and can reduce the increase of two-phase friction pressure drop per unit length with the increase of heat flux. Under electric field-free conditions, the porous phase separation structure exhibits a maximum increase in heat transfer efficiency of 26.2% compared to conventional structures. Furthermore, the maximum reduction in two-phase frictional pressure drop per unit length reaches 19.4%. In the presence of the synergistic effect between an electric field and the phase separation structure, the maximum heat transfer efficiency is enhanced by 65.5%. The above analysis provides an effective means for investigating the comprehensive performance of phase separation structures and the bubble dynamics of confined bubbles in heat sinks.

Experimental investigation on condensation heat transfer coefficient and frictional pressure drop of low GWP refrigerant R1234yf on a wavy fin surface

Refrigerant R1234yf is an ideal substitute to replace R134a due to its lower value of global warming potential (GWP). The thermophysical properties of R1234yf are similar with R134a and thus it is considered as a potential replacement for R134a in thermal fluid systems. Further, it was noticed that there is a lack of analysis on the condensation of R1234yf in a brazed plate fin compact heat exchanger (CHE) employing wavy fins. In the present work, condensation heat transfer coefficient and frictional pressure drop of refrigerant R1234yf are investigated in a brazed plate fin compact heat exchanger with wavy fin surfaces. The test condenser with wavy fin surface was selected to examine the condensation heat transfer coefficient and frictional pressure drop using refrigerants R134a and R1234yf for varying mass flux and saturation temperatures. The present study also discusses the dependency of specific kinetic energy of refrigerant on frictional pressure drop. Further, using the experimental data of wavy fin surfaces, suitable correlations for the condensation heat transfer coefficient and frictional pressure drop of R1234yf were proposed. The present paper also reports the comparison made between refrigerants R134a and R1234yf. Comparison between R1234yf and R134a shows that condensation heat transfer coefficient and frictional pressure drop of R1234yf is 9–16 % and 10–17 %, respectively, lower than of R134a in the mass flux range 15–44 kg/m2s.

Development of Two-Phase Frictional Pressure Gradient Correlation for Saturated Cryogenic Flow Boiling in Uniformly Heated Tubes

Experiments to determine two-phase frictional pressure drop for saturated cryogenic flow boiling in uniformly heated round tubes have been performed throughout the globe during the past sixty years for a variety of cryogens. However, the experimental data are scarce and either rarely published or published only for a few cryogens, with majority of the data remaining in the archives of authors, or in obscure technical reports of an organization or other inaccessible sources. In the present study, the Purdue University-Boiling and Two-Phase Flow Laboratory (PU-BTPFL) Cryogenic Total Pressure Drop Database is consolidated both for saturated adiabatic liquid-vapor and saturated flow boiling for cryogens from world literature dating back to 1964. With 474 total pressure drop data points for LHe, LH2, LNe, and LN2, it represents the largest total pressure drop database for cryogenic saturated two-phase flow assembled to date. Using this database, predictive accuracy of seminal frictional pressure gradient correlations employing both the Homogenous Equilibrium Model (HEM) and the Separated Flow Model (SFM) are assessed. The distinct fluid physics unique to cryogens is studied further using the database and new dimensionless criteria are developed to demarcate a dispersed flow dominant data subset from the separated flow dominant data subset, which are better represented using HEM and SFM, respectively. These criteria are subsequently used to develop a hybrid HEM-SEM frictional pressure gradient correlation for saturated cryogenic flow for the two distinct data subsets. The new hybrid correlation performs the best of all seminal correlations and provides excellent predictive agreement with the database, evidenced by a Mean Absolute Error (MAE) of 14.7 %.

Condensation Performance of Symmetrical and Parallel Capsule Plate Heat Exchangers

Condensation characteristics of symmetrical and parallel capsule plate heat exchangers were experimentally investigated in this article. Plate heat exchangers with parallel capsule corrugation, symmetrical capsule corrugation, and chevron-type corrugation are manufactured as the test sections, each containing 41 plates. Pure steam and water are the hot and cold fluids, respectively. The experiments covered a range of equivalent Reynolds number from 9,000 to 25,000, mass flux of condensing fluid from to 18 to 49 kg/(m2·s), liquid Prandtl number from 1.37 to 1.71, average vapor quality from 0.5 to 0.7, and condensation pressure from 110 to 240 kPa. The correlations of condensation heat transfer coefficient and friction factor of the symmetrical and parallel capsule plate heat exchangers are developed. Compared with chevron-type plate heat exchangers, capsule plate heat exchangers show low friction pressure drop, and 33% higher energy efficiency index for condensation flow and heat transfer. Specifically, the energy efficiency index of the symmetrical capsule plate heat exchanger is 8.6% higher than that of the parallel capsule plate heat exchanger, indicating the superiority of counter-rotating longitudinal vortices over co-rotating longitudinal vortices in condensation, similar to what is observed in single-phase flow and heat transfer.

Heat Transfer and Entropy Generation of Turbulent Flow in Corrugated Channel using Nanofluid

This paper presents a numerical study of two-dimensional, turbulent flow with heat transfer in a corrugated channel using 3% of Al2O3 nanoparticles volume fraction dispersed in a basic fluid (water) with a particle diameter of 30nm. The equations governing the flow are: the energy equation to model heat transfer, the momentum equations and the continuity equation. To simulate different values ​​of the Reynolds number ranging from 1000 to 6000, the CFD code Ansys- Fluent based on the finite volume method is used. For turbulence modeling, the k-ε realizable model was used. The results are validated by other scientific work. Nu, skin friction coefficient, pressure drop and PEC factor are explored. We noted that the increase in the Reynolds number leads to an increase in the average Nusselt and a decrease in the coefficient of friction. Particular attention is paid to the generation of transfer entropy, friction and general entropy as well as the Bejan number. The transfer entropy generated in the fluid crossing the channel as well as the Bejan number decrease with the increase of the Reynolds while that of the friction increases. In addition, the use of nanofluid (Al2O3/water) gives better heat transfer than pure water.

Flow boiling heat transfer characteristics of zeotropic refrigerant/oil mixtures in horizontal smooth tube

In the Organic Rankine Cycle (ORC), the lubricant is inevitably mixed with the working fluid, which affects its flow boiling heat transfer characteristics in the evaporator. In this study, the flow boiling heat transfer characteristics of zeotropic fluid R141b/R245fa at different polyester oil (POE) concentrations were experimentally investigated. Meanwhile, the effects of heat flux, mass flux and vapor quality were also investigated. The results show that both the mass flux and vapor quality can increase heat transfer coefficient (HTC) and frictional pressure drop (FPD), while the heat flux decreases the HTC and increase the FPD. The maximum promotion of HTC is achieved at an oil concentration of 2.3% compared to other concentrations. Except for the oil concentration of 3%, the FPD of R141b/R245fa-POE mixture increases with the oil concentration. Additionally, the effects of lubricant oil on the HTC and FPD of refrigerant/POE mixture were discussed. The Schager-R22/150-SUS correlation can well predict the enhancement factor of HTC and penalty factor of FPD. The comprehensive factor is always greater than 1, so POE can enhance the overall heat transfer performance of refrigerant-POE mixture.

Hydrocarbon condensation flow and heat transfer in wave helical pipes: Effect of structure parameters

In this study, two types of wave-structured pipes for heat exchangers that are easy to process are proposed, and the condensation pressure drop and heat transfer performances for hydrocarbon refrigerant upward flow in wave helical pipes are investigated. A numerical method based on the inhomogeneous two-fluid model is introduced and validated by experimental data in literature. The effect of wave parameters (wave height and wave length) on heat transfer, pressure drop and flow patterns are studied. At lower vapor quality conditions, it is found that as wave height increases, frictional pressure drop and heat transfer coefficient exhibit a trend of initially increasing, followed by decreasing, and then increasing again. However, under higher vapor quality conditions, frictional pressure drop and heat transfer coefficient continue to increase. Besides, both the frictional pressure drop and heat transfer coefficient increase with the decrease of wave length. The averaged enhancement on frictional pressure drop for vertical wave pipes and horizontal wave pipes are respectively 1.279–5.148 and 1.238–5.808 times, while the average augmentation in heat transfer coefficient is 1.243–3.006 and 1.316–3.579 times, respectively. The comprehensive thermal performance of wave pipes was assessed using the performance evaluation criterion. The range of averaged performance evaluation criteria (under different vapor qualities) in vertical wave pipes and horizontal wave pipes are respectively 1.150–1.737 and 1.230–2.026. The horizontal wave pipe when the wave height is 5 mm and wave length is 5.24 mm shows the best overall thermal performance, with an averaged performance evaluation criterion of 2.026. The two types of wave-structured pipes proposed in this paper can increase the pipe length of the spiral-wound heat exchanger at the same volume, while also improving the performance of the spiral-wound heat exchanger. The outcomes of the study can offer some assistance in creating simple but effective spiral wound heat exchangers.

Flow boiling heat transfer characteristics of zeotropic mixture CO2/R152a with large temperature glide in a 2 mm horizontal tube

The flow boiling characteristics of CO2/R152a zeotropic mixtures with three mass fractions (22.1/77.9, 47.4/52.6 and 65.3/34.7 wt.%) and the pure components in a 2 mm horizontal tube are experimentally studied. The flow boiling characteristics are tested in the range of temperature glide of 30.9∼38.3 °C, mass flux of 200∼400 kg/m2s, heat flux of 12∼36 kW/m2, and saturation temperature of 5∼15 °C. Flow patterns are observed and the flow pattern map is also described, and the heat transfer and pressure drop characteristics of working fluid are also conducted. Furthermore, a new correlation to predict the heat transfer coefficient of large temperature glide working fluid is proposed. The results show 7 types of flow patterns are recorded, showing the flowing characteristic between the conventional and microscale channel. The heat transfer coefficient of CO2/R152a increases slowly with vapor quality, and decrease rapidly when vapor quality reaches to the initial dryout vapor quality, which is lower than that of pure CO2. The mass transfer resistance and sensible heat resistance shows a severe deterioration effect on the flow boiling heat transfer due to the high temperature glide as the sensible heat accounts for 13.12∼15.44 % for the mixture CO2/R152a. The frictional pressure drop gradient of CO2/R152a increases with mass flux, and decreases with the increase of saturation temperature and mass fraction of CO2. Considering the mass transfer resistance and effects of non-negligible sensible heat contribution, a new heat transfer correlation is proposed, with the mean absolute deviation of 16.9 %, which shows an acceptable prediction accuracy compared with the reported data in the literatures.

Simulation of vertical core-annular flow with a turbulent annulus

The Reynolds-Averaged Navier Stokes (RANS) with the Launder & Sharma low-Reynolds number k−ε model was used to simulate core-annular flow in the same configuration with vertical upflow as considered by Kim & Choi (2018), who carried out Direct Numerical Simulations (DNS), and by Vanegas Prada (1999), who performed experiments. The DNS are numerically very accurate and can thus be used for benchmarking of the RANS turbulence model. There is a large ratio between the oil and water viscosities, and the density difference between the water and oil is only small. The frictional pressure drop was fixed and the water holdup fraction was varied. Differences between the RANS and DNS predictions, e.g. in the wave structure and in the Reynolds stresses, are discussed. Despite the shortcomings of the considered Launder & Sharma low-Reynolds number k−ε model in RANS, in comparison to DNS, the RANS approach properly describes the main flow structures for upward moving core-annular flow in a vertical pipe, like the travelling interfacial waves in combination with a turbulent water annulus. The Fanning friction factor with RANS is 18% lower than with DNS, and the holdup ratio with RANS is only slightly higher than with DNS (i.e. it has a slightly larger tendency to accumulate water in RANS than in DNS).

Two-phase flow structures in a helically coiled microchannel: An experimental investigation

At the microfluidic scale, the utilization of helically coiled channels (HCCs), also known as a spiral channel, for two-phase flow offers numerous advantages in various applications. Existing articles mainly focus on the macro-scale transport, examining secondary flows induced in curved channels. The increasing demand, however, for innovative miniature equipment for thermal energy management emphasizes the importance of comprehending gas–liquid micro-scale flow in curved channels. Unfortunately, despite a vast body of literature on this paradigm, there is still a lack of systematic investigations into the underlying facets of two-phase micro-scale transport in HCCs. To address this gap, our study conducted experiments on adiabatic two-phase air–water flow inside an up-flow helical micro-scale tube. The tube had a hydraulic diameter of 0.87 mm, a coil diameter of 50 mm, and a helical pitch of 20 mm. The primary aim was to explore the impact of centrifugal force on flow pattern, void fraction, and frictional pressure drop characteristics. Additionally, we carefully examined the phase separation phenomenon influenced by the secondary flows induced by the curved channel. In particular, we compared the gas-core flow pattern (either throat-annular flow or annular flow), void fraction, and frictional pressure drop obtained from our experiments on the helical tube with corresponding results based on straight micro-scale channel configurations for an Eötvös number of approximately 0.01. In summary, this study delves deep into the crucial aspects of two-phase micro-scale transport in HCCs, contributing to a better understanding of these systems for future advancements in micro-channel applications.

Evaluation of frictional pressure drop correlations for air-water and air-oil two-phase flow in pipeline-riser system

Accurate prediction of the frictional pressure drop is important for the design and operation of subsea oil and gas transporting system considering the length of the pipeline. The applicability of the correlations to pipeline-riser flow needs evaluation since the flow condition in pipeline-riser is quite different from the original data where they were derived from. In the present study, a comprehensive evaluation of 24 prevailing correlation in predicting frictional pressure drop is carried out based on experimentally measured data of air-water and air-oil two-phase flows in pipeline-riser. Experiments are performed in a system having different configuration of pipeline-riser with the inclination of the downcomer varied from −2° to −5° to investigated the effect of the elbow on the frictional pressure drop in the riser. The inlet gas velocity ranges from 0.03 to 6.2 m/s, and liquid velocity varies from 0.02 to 1.3 m/s. A total of 885 experimental data points including 782 on air-water flows and 103 on air-oil flows are obtained and used to access the prediction ability of the correlations. Comparison of the predicted results with the measured data indicate that a majority of the investigated correlations under-predict the pressure drop on severe slugging. The result of this study highlights the requirement of new method considering the effect of pipe layout on the frictional pressure drop.

Effect of surface engineering methods on refrigerant evaporating flow characteristics in annular tubes: Experimental approach

Techniques such as mechanical, electrochemical, and chemical etching are recognized as functional strategies for modifying surface roughness, which in turn manipulates surface energy. This research aims to delve into the influence of these factors on the heat transfer and pressure drop characteristics of two-phase R134a refrigerant flow during boiling inside horizontal annular concentric tubes with internal heat flux. The study scrutinizes the heat transfer coefficient and frictional pressure drop for R134a evaporating flow in an annular tube featuring an inner diameter of 28.6 mm and an outer diameter of 42 mm. The exploration spans a broad range of parameters, which include vapor quality from 0.15 to 0.8, mass flux from 6.72 to 20.16 kg/m2s, internal heat flux from 0.608 to 2.432 kW/m2, and varying surface characteristics. The results show that escalating surface roughness via mechanical techniques introduces greater resistance to flow shear stress in comparison to other methods. Moreover, an increase in surface roughness contributes to a reduction in contact angle and an expansion of the heating area, thereby enhancing both heat transfer coefficient and pressure drop. However, the implementation of low-energy coating holds an insignificant influence on R134a heat transfer characteristics, potentially due to its diminutive surface tension. This results in minor variations in the refrigerant contact angle in relation to the coatings. Performance evaluation criteria have been employed to assess the optimal thermal performance of the surface engineering method, with peak performance achieved at a mass flux of 20.16 Kg/m2s and an internal heat flux of 2.432 kW/m2. Ultimately, the study corroborates the experimental procedure by comparing the observed outcomes with established empirical correlations for R134a flow boiling.

Boiling heat transfer and pressure drop of R290 in a micro-fin tube

The present study focuses on the analysis of heat transfer and pressure drop of R290 in a horizontal micro-fin tube. In this regard, experiments were conducted, and flow patterns, heat transfer coefficient, and friction pressure drop were analyzed under various conditions. A wide range of conditions was studied, including saturation pressures ranging from 0.215 to 0.415 MPa, mass fluxes ranging from 70 to 190 kg m−2 s−1, heat fluxes in the range of 10.6–73.0 kW m−2, and vapor qualities from 0 to 0.96. The performed analyses revealed four flow patterns, including bubbly flow, plug flow, slug flow, and annular flow. It is found that the onset of dry-out occurs when the liquid film thickness is equal to the fin height and occurs earlier at high heat fluxes and low saturation pressures. The maximum heat transfer coefficient and friction pressure drop occur at the onset of dry-out, and the heat transfer coefficient exhibits a significant correlation with the heat flux. The obtained results were verified using the heat transfer correlation of Cavallini and the friction pressure drop correlation of Rollman, where the average relative deviation (ARD) was −24.01% and −8.65%, respectively.

Practical design guidelines for heat transfer enhancement of condensers

Enhanced condensation is of great practical interest to further improve and miniaturize power generation and refrigeration systems. Studies in the literature that investigate enhanced condensation focus on increasing the heat flux at high vapor qualities; however, many thermodynamic cycles such as vapor compression and the Rankine cycle require complete condensation for cycle operation, and in such cases, low-quality condensation can be the limiting factor. Performance evaluation criteria (PEC) can be used to assess the relative improvement achieved by different enhancement methods, but most PEC are only applicable to single-phase flows. The present study therefore develops local design guidelines for condensing flows to determine the regions of the condenser that most need heat transfer enhancement. This is achieved by comparing the relative benefits and penalties associated with enhancing low-quality, medium-quality, and high-quality condensation equally. The effects of varying heat transfer coefficient, frictional pressure drop, temperature reduction due to pressure drop, driving temperature difference, and coupling fluid thermal resistance are considered. Three enhancement techniques are then analyzed with these approaches to determine their relative benefits and disadvantages. Cases that consider fixed geometries, fixed flow areas, and variable flow area are considered, along with comparing the relative benefits of size reduction, increased heat duty, and reduced temperature difference. Results show that heat transfer enhancement is achieved when the increase in heat exchanger conductance exceeds the reduction in the heat exchanger temperature difference induced by condensation pressure drop. Additionally, it is found that enhancement is typically most needed in the low vapor quality region of the condenser, except in situations where temperature pinching occurs along the heat exchanger.

Numerical investigation of flow characteristics and heat transfer efficiency in sawtooth corrugated pipes with Al2O3-CuO/Water hybrid nanofluid

Improvements in heat transfer technologies are a key factor in making all industries that use condensers, solar collectors, and thermal machines more efficient and use less energy. With the development of corrugated pipes that improve convection in an affordable way, passive techniques to optimise heat conduction have entered the market. The objective of this study was to evaluate the performance of a hybrid nanofluid in improving heat transfer efficiency by using corrugated pipes. In particular, we investigated the behaviour of a hybrid nanofluid with a volume concentration of 0.5% Al2O3-CuO/water (80:20 by volume). To investigate the characteristics of the hybrid nanofluid, we used sawtooth-shaped corrugated pipes with different pitch height and pitch distance. The results show that increasing the Reynolds number improves the heat transfer due to higher flow velocities and disturbances from the corrugated surfaces. In terms of the Nusselt number, which is an indicator of convective heat transfer, corrugated pipes consistently perform better than smooth pipes. The average Nusselt number increases between 8% and 74% for the sawtooth-shape corrugated pipes which depends on the pitch height, pitch distance, and Re. However, the presence of corrugations leads to increased friction factors and pressure drops compared to smooth pipes, with larger corrugation amplitudes exacerbating these effects. Shorter pitch distances result in higher Nusselt numbers due to increased flow turbulence and mixing, while longer pitch distances lead to lower Nusselt numbers. These results contribute to the understanding of the effects of pitch height and pitch spacing on corrugated pipe performance. They also provide insights for the design optimisation of efficient heat transfer systems.