Inlet Flow Conditions Research Articles

Experimental investigation on flow boiling characteristics of radial expanding minichannel heat sinks applied for two-phase flow inlet

Generally, two-phase (TP) flow inlet condition is not allowed for conventional microchannel heat exchangers, and a certain subcooled degree of liquid is strictly required to guarantee the efficient operation of system. To solve multi-heat sources cooling issue, a type of circular radial expanding minichannels heat sink (REMHS) utilizing flow boiling of deionized water is proposed in present work. Flow boiling heat transfer of REMHS is investigated with the inlet vapor mass qualities in the range of 0.01–0.53. Under TP flow inlet, slug-annular and annular flows are the main flow patterns observed in REMHS in this work, where the low-medium mass flux (22–132 kg/m2s) and medium-high heat flux (33.1–176.4 kW/m2) conditions are applied. The experimental results illustrate that the proposed REMHS under TP flow inlet maintains a stable and high flow boiling heat transfer performance without occurring heat deterioration. When xin increases from 0.04 to 0.53, the maximum local heat transfer coefficient increases by 22% from 42.5 to 51.9 kW/m2K. The flow distribution among the expanding channel array is relatively uniform and the heat transfer capability in the REMHS displays a good symmetry under TP flow inlet. The maximum discrepancy in heat transfer coefficient is below 19% within our experimental range. Six classic flow boiling heat transfer correlations are compared with the experimental results, and Li & Wu's correlation gives a good prediction on all the measured data with MAE of 21.1%. Besides, the thermal independence of REMHS connected in series is also examined, and the mutual interaction in heat transfer between the REMHS in series is trivial, the proposed REMHS is expected to be applied to multi-heat source cooling.

Spray drying experiments and CFD simulation of guava juice formulation

This paper presents a computational fluid dynamics (CFD) approach to model the spray-drying of a formulation of guava juice. The computational model incorporates the characteristic drying rate curve (CDC) of the formulation. The CDC is derived from the drying history of single droplets suspended on a thin glass filament. The computational domain is based on the geometry of a small production spray-dryer with a capacity of 10 kg/h. A two-phase Eulerian-Lagrangian model implemented in a commercial CFD package (ANSYS-FLUENT) is used to simulate the gas-particle interaction in the dryer. Hot-wire anemometry (HWA) measurements of the air flow in the drying chamber are used to calibrate the CFD model. The URANS k-ω SST and the Scale-Adapted Simulation turbulence models are employed to compute the unsteady three-dimensional flow field. Model calibration suggests that the swirl number of the air flow entering the dryer is about 0.61. Simulations predict high axial velocities along the center-line and a narrow jet core in the upper part of the dryer. Lagrangian particle tracking is used to calculate the particle trajectories starting from the perimeter of the rotary disk atomizer and ending at the dryer walls and bottom outlet. Simulation results of particle size distribution, time-average temperature and humidity profiles are presented in order to analyze the overall drying performance. The procedure shown in this work emphasizes the need of implementing drying kinetics with experimental support, and suitable inlet flow conditions to perform detailed CFD simulations of the spray-drying of fruit juices.

Hemodynamic Analysis of a Microanastomosis Using Computational Fluid Dynamics.

Technical issues in free flap transfer, such as the selection of recipient vessels and the positioning and method of anastomosis of the vascular pedicle, have been the subject of vigorous debate. Recent developments in computational fluid dynamics (CFD) have enabled the analysis of blood flow within microvessels. In this study, CFD was used to analyze hemodynamics in a microanastomosis. In the fluid calculation process, the fluid domain modelizes microvessels with anastomosis. The inlet flow conditions were measured as venous waveform, and the fluid is simulated as blood. Streamlines (SL), wall shear stress (WSS), and oscillatory shear index (OSI) at the anastomosis were visualized and analyzed for observing effects from the flow field. Some flow disruption was evident as the SL passed over the sutures. The maximum recorded WSS was 13.37 Pa where the peak of a suture was exposed in the lumen. The local maximum value of the OSI was 0.182, recorded at the base of the anastomosis on the outflow side. In the ideal anastomosis, the SL is disrupted as little as possible by the sutures. The WSS indicated that thrombus formation is unlikely to occur at suture peaks, but more likely to occur at the base of sutures, where the OSI is high. Tight suture knots are important in microanastomosis.

Experimental and Numerical Investigation of Swirling Flow on Triple Elbow Pipe Layout

The secondary flow downstream of a triple elbow layout was studied experimentally and numerically to visualize the flow behavior under swirling inlet flow conditions. The inlet swirling condition was generated by a swirl generator, consisting of a rotary pipe and honeycomb assembly. The experiments were carried out in turbulent water flow condition at Reynolds number Re = 1 × 104 and inlet swirl intensity S = 1. Ultrasonic measurements were taken at four locations downstream of the third elbow. The two-dimensional velocity field of the flow field was measured using the phased array ultrasonic velocity profiler technique to evaluate the flow field with separation. Furthermore, a numerical simulation was performed and its results were compared with the experimental data. The numerical result was obtained by solving three-dimensional, Reynolds-averaged Navier-Stokes equations with the renormalization group k-ε turbulence model. The experimental results confirmed that the swirling flow condition modified the size of the separation region downstream of the third elbow. A qualitative comparison between the experimental and CFD simulation results of the averaged velocity field downstream of the third elbow showed similar tendency on reverse flow.

CFD analysis of conical diffuser under swirl flow inlet conditions using turbulence models

Diffuser are used to maximize static pressure recovery in flow systems such as turbo-machines, turbine exit, duct flows, aircraft engine inlets, closed circuit wind tunnels etc. Many of turbo-machines has swirl flow at inlet which requires thorough investigation to predict fluid flow parameters like velocity, pressure and mass flow. The current research investigates the conical diffuser subjected to swirl flow inlet conditions using Computational Fluid Dynamics. Different cone angles of diffuser are considered for analysis i.e. 10°, 20° and 30°. The CAD model of diffuser is developed using Creo 2.0 software and CFD analysis is conducted using RNG k-epsilon turbulence models using ANSYS CFX software. The results generated from CFD analysis has thrown light on effect of diffuser cone angles with respect to outlet pressure, velocity and turbulence kinetic energy. The research shows that cone angle has significant effect on static pressure recovery of diffusers.

Investigation into the outlying swirl instability in the hydro-turbine draft tube under part-load operation

The swirling trajectory of the draft tube flow improves the pressure recovery. Resultant flow instabilities are still problematic for an optimized hydropower generation. Engineering solutions are getting increasing importance. However; the depth of wall-mounted countermeasures is a challenging parameter. Performance assessment of countermeasures would rate dampened pressure pulsations in terms of their sources so as to place them at a proper location. A method for evaluating the instability in the outlying domain of the draft tube cone is herein suggested. Two part-load operating points with opposite polarity in pressure pulsations have been investigated. The numerical approach of Shear Stress Transport has been used, and experimental agreement of measurements for the pressure spectra is verified at the wall. Inlet flow conditions and the growth of the core instability are described. The outlying domain of the draft tube has been found to be composed of three distinct zones of influence: the shortest upper cone with upstream travelling influences, the longest middle cone with core excitation, and the lowermost cone dominated by the backward elbow influence. The wall response lies within a definite high-frequency range proper to the operating point. During admission, the frequency range becomes even wider due to high kinetic energy and the highest pressure amplitude of the draft tube is developed by the impact of blade passing frequency. The excitation from the vortex rope precession in the outlying domain strongly depends on the operating condition. Thus, further works should extend this study over a wider operation range, and mount pressure sensors on countermeasures in order to identify their own instability contribution.

Experimental visualization of flow structure inside subchannels of a 4 × 6 rod-bundle

A fuel element geometry frequently used in nuclear reactors is the rod-bundle. In it, coolant flows axially through subchannels formed between the rods. The mixing of cooling fluid in a rod-bundle reduces the temperature differences in the coolant and along the perimeter of the rods. To ensure thermal performance of a nuclear reactor, detailed information about heat transfer and turbulent mixing taking place within subchannels is required. To improve the prediction ability of the subchannel analysis using computer code, a considerable amount of experimental data should be utilized for developing a new model for subchannel analysis. Experimental work has been conducted in the PRIUS-I (in-PWR Rod-bundle Investigation of Undeveloped mixing flow across Subchannel) test facility, which has adopted the matched index-of-refraction (MIR) technique to visualize the multi-dimensional velocity and turbulence fields in an unheated 4 × 6 rod-bundle geometry as well as to provide benchmark data for computer code validation. The 4 × 6 rod-bundle array has the same pitch-to-diameter (P/D) as the prototype fuel assembly. PRIUS-I test facility can simulate the uniform and non-uniform inlet flow conditions in order to validate the effect in crossflow between the fuel assemblies of PWR. In this study, the crossflow and mixing phenomena that exist between two adjacent subchannels through the gaps are visualized in any cross section of the rod-bundle geometry using a MIR technique. For non-uniform inlet velocity condition, the flow characteristics near the inlet where the flow mixing occurs actively is compared with computational fluid dynamics (CFD) calculation results. The experimental data can be used as benchmark data for the system safety analysis code or CFD code validation.

6125High wall shear stress predicts plaque rupture of the aortic arch: computational fluid dynamics model and non-obstructive general angioscopy study

Abstract Background Wall shear stress (WSS) has been considered as a major determinant of aortic atherosclerosis. Recently, non-obstructive general angioscopy (NOGA) was developed to be able to visualize a variety of its atherosclerotic pathology, including in vivo ruptured plaque (RP) in the aorta. We, therefore, investigated the relationship between NOGA derived RP in the aortic arch and the stereographic distribution of WSS by using computational fluid dynamics modeling (CFD) on three-dimensional CT angiography (3D-CT). Methods We investigated 30 consecutive patients who underwent 3D-CT before and NOGA during coronary angiography. WSS in the aortic arch was measured with an application of CFD based on finite element method by using uniform inlet and outlet flow conditions. Aortic RP was detected by NOGA. Results The maximum and mean values of WSS were 67.2±29.2 Pa and 2.4±0.6 Pa. A total of 18 RPs was detected by NOGA. The patients with a distinct RP showed a significantly higher maximum WSS in the whole aortic arch, and the greater and lesser curvature of the aortic arch than those without it (73.3±29.0 Pa vs 50.4±15.2 Pa, p=0.035, 95.0±27.5 Pa vs 42.8±25.2 Pa, p=0.003, 70.8±29.3 Pa vs 46.1±11.9 Pa, p=0.013, respectively), whereas there was no significant difference in the mean WSS between those with and without it. In a multivariate analysis, the maximum value of WSS was an independent predictor of RP in the aortic arch (odds ratio 1.05, 95% confidence interval 1.01–1.13, p=0.019). Representative picture of WSS and NOGA Conclusions Aortic RP detected by NOGA was strongly associated with the higher maximum WSS in the aortic arch derived by CFD using 3D-CT. Maximum WSS may explain the underlying mechanism of not only aortic atherosclerosis, but also aortic RP.

Computational Fluid Dynamics-Based Blood Flow Assessment Facilitates Optimal Management of Portal Vein Stenosis After Liver Transplantation

BackgroundPortal vein stenosis develops in 3.4–14% of split liver transplantation1–3 and its early detection and treatment are essential to achieve long-term graft survival,2–5 although the diagnostic capability of conventional modalities such as Doppler ultrasound and computed tomography is limited.1,4,5 MethodsThis study used computational fluid dynamics to analyze portal vein hemodynamics in the management of post-transplant portal vein stenosis. To perform computational fluid dynamics analyses, three-dimensional portal vein model was created using computed tomographic DICOM data. The inlet flow condition was set according the flow velocity measured on Doppler ultrasonography. Finally, portal vein flow was simulated on a fluid analysis software (Software Cradle, Japan). ResultsAn 18-month-old girl underwent liver transplantation using a left lateral graft for biliary atresia. At the post-transplant 1-week evaluation, the computational fluid dynamics streamline analysis visualized vortices and an accelerated flow with a velocity ratio < 2 around the anastomotic site. The wall shear stress analysis revealed a high wall shear stress area within the post-anastomotic portal vein. At the post-transplant 6-month evaluation, the streamline analysis illustrated the increased vortices and worsening flow acceleration to reach the proposed diagnostic criteria (velocity ratio > 3:1).3,5 The pressure analysis revealed a positive pressure gradient of 3.8 mmHg across the stenotic site. Based on the findings, the patient underwent percutaneous transhepatic portal venoplasty with balloon dilation. The post-treatment analyses confirmed the improvement of a jet flow, vortices, a high wall shear stress, and a pressure gradient. DiscussionThe computational fluid dynamics analyses are useful for prediction, early detection, and follow-up of post-transplant portal vein stenosis and would be a promising technology in post-transplant management.

Numerical simulation of buckling and asymmetric behavior of flexible filament using temporal second-order immersed boundary method

PurposeThe purpose of this paper is to perform two-dimensional numerical simulation involving fluid-structure interaction of flexible filament. The filament is tethered to the bottom of a rectangular channel with oscillating fluid flow inlet conditions at low Reynolds number. The simulations are performed using a temporal second-order finite volume-based immersed boundary method (IBM). Further, to understand the relation between different aspect ratios i.e. ratio of filament length to channel height (Len/H) and fixed channel geometry ratio, i.e. ratio of channel height to channel length (H/Lc) on mixing and pumping capabilities.Design/methodology/approachThe discretization of governing continuity and Navier–Stokes equation is done by finite-volume method on a staggered Cartesian grid. SIMPLE algorithm is used to solve fluid velocity and pressure terms. Two cases of oscillatory flow conditions are used with the flexible filament tethered at the center of bottom channel wall. The first case is sinusoidal oscillatory flow with phase shift (SOFPS) and second case is sinusoidal oscillatory flow without phase shift (SOF). The simulation results are validated with filament dynamics studies of previous researchers. Further, parametric analysis is carried to study the effect of filament length (aspect ratio), filament bending rigidity and Reynolds number on the complex deformation and behavior of flexible filament interacting with nearby oscillating fluid motion.FindingsIt is found that selection of right filament length and bending rigidity is crucial for fluid mixing scenarios. The phase shift in fluid motion is also found to critically effect filament displacement dynamics, especially for rigid filaments. Aspect ratio, suitable for mixing applications is dependent on channel geometry ratio. Symmetric deformation is observed for filaments subjected to SOFPS condition irrespective of bending rigidity, whereas medium and low rigidity filaments placed in SOF condition show severe asymmetric behavior. Two key findings of this study are: symmetric filament conformity without appreciable bending produces sweeping motion in fluid flow, which is highly suited for mixing application; and asymmetric behavior shown by the filament depicts antiplectic metachronism commonly found in beating cilia. As a result, it is possible to pin point the type of fluid motion governing fluid mixing and fluid pumping. The developed computational model can, thus, successfully demonstrate filament-fluid interaction for a wide variety of similar problems.Originality/valueThe present study uses a temporal second-order finite volume-based IBM to examine flexible filament dynamics for various applications such as fluid mixing. Also, it highlights the relationship between channel geometry ratio and filament aspect ratio and its effect on filament sweep patterns. The study further reports the effect of filament displacement dynamics with or without phase shift for inlet oscillating fluid flow condition.

Experimental analysis of SiO2-Distilled water nanofluids in a Polymer Electrolyte Membrane fuel cell parallel channel cooling plate

An experimental report on the thermal performance of Silicone Dioxide (SiO2) nanofluid coolants based on a PEM fuel cell cooling system is presented. The aim of this study is to evaluate the feasibility of applying these nanofluids coolants as an alternative to conventional distilled water through detailed analysis of thermofluids behaviour in a simulated cooling plate environment. SiO2 nanoparticles were dispersed in distilled water at 0.1%, 0.3% and 0.5% volume concentrations and tested in a parallel channel cooling plate system. A constant heat load was supplied to simulate a fuel cell stack thermal condition. At inlet flow conditions from 750 to 900 Reynolds number, the SiO2 nanofluids reduced the average plate temperatures by 15%–20% compared to conventional water coolant. The nanofluids also increased the cooling effectiveness by a similar margin, as well as improving the bulk heat transfer coefficient to a range between 2700 and 4400 W m−2. oC−1. However, the required pumping power was also increased due to the added viscous effect. Through the Advantage Ratio (AR) analysis, it was concluded that the enhancement in heat transfer mechanics was more significant than the penalties in fluid flow dynamics. Thus, the SiO2 nanofluids and the cooling plate design are possible options for advanced PEM fuel cell thermal management practice in future stack designs.

Relation between oil-water interfacial flow structure and their separation in the oil-water mixture flow in a curved channel: An experimental study

We investigate the oil-water separation in a curved channel under a range of inflow conditions, focusing on the fundamental multiphase flow physics occurring at the oil-water interface. With a silicone oil (viscosity: 10 mm2/s, density: 935 kg/m3), we perform a series of water-tunnel experiments to measure the evolution of oil-water mixture flow inside a curved channel, devised for realizing a continuous and effective oil recovery (separation). The oil-water mixture velocity at the channel inlet is 1.35–2.35 m/s (Reynolds and Froude numbers based on the inlet height and length of the channel are 0.6–2.0 × 104 and 0.14, respectively), and we vary the inlet oil volume fraction as 0.49–0.85. We find that the efficiency of oil separation is strongly affected by the oil-water interfacial flow structures and identify two typical flow patterns: wavy oil-water interface and dispersed-oil flow. When the inlet oil fraction is high ( > 0.74), the instability occurring at the wavy oil-water interface plays a dominant role in determining the oil recovery rate (which is as high as 80% in general). As the inlet oil fraction becomes smaller ( < 0.7), on the other hand, the oil dispersion starts to appear vigorously, which interferes with the oil separation process and the recovery rate drops below 60% at the oil fraction smaller than 0.5. Through the quantitative analysis of the optically measured oil-water interface phenomena, we suggest theoretical models to predict the oil recovery rate as a function of the interfacial fluctuation of wavy oil-water interface and the fraction of dispersed oil phase, depending on the inlet flow conditions. We also propose a strategy to maximize the oil recovery rate of large-scale oil-water separation device, which is expected to be beneficial in many applications.

Effect of carotid artery stenosis on neck skin tissue heat transfer

The buildup of plaque in the carotid artery leads to the narrowing of the arterial lumen, known as stenosis. Arterial stenosis affects the supply of blood to the brain and may result in stroke if the plaque is dislodged. This study aims to evaluate stenosis influenced hemodynamics of the carotid artery and its effect on the bio-heat transfer to the external neck skin surface. Systolic and Diastolic carotid artery flow inlet conditions, using a finite volume numerical scheme, are simulated with the inclusion of varied stenosis degree in the carotid model (25%–90%). A 3-dimensional (3D) conjugate heat transfer study is performed on an idealized carotid artery geometry, encapsulated in a tissue model resembling the human neck. With no significant difference in the average external neck skin temperature, variation in the temperature contours is observed between stenosed and non-stenosed cases. With the increase in the carotid artery stenosis, a quantitative change, the presence of a colder region called ‘cold feature’, in external neck skin temperature features is reported. This numerical study will prove to be a strong basis for future patient-specific evaluations.

Numerical study of convective heat transfer of a supersonic combustor with varied inlet flow conditions

Characteristics of convective heat transfer of a supersonic model combustor with variable inlet flow conditions were studied by numerical simulation in this paper. The three-dimensional flow and wall heat flux at different air inlet Mach numbers of 2.2, 2.8 and 3.2 were studied numerically with Reynolds-averaged Navier–Stokes equations with a shear-stress transport (SST) k − ω turbulence model and a three-step reaction model. Meanwhile, ethylene was chosen as the fuel, and the fixed fuel-to-air equivalence ratio is 0.8 in all cases in this paper. The results of the simulations indicate that wall heat flux distribution of the combustor is very non-uniform with several peaks of wall heat flux at varied locations. For the low inlet Mach number of 2.2, a shock train structure is formed in the isolator, and three peaks of wall heat flux are located respectively on the backward face of the cavity, on the side wall near the fuel injection and on the bottom wall near the injection holes, and a maximum wall heat flux reaches 5.4 MW/m2. For the medium inlet Mach number of 2.8, there exists a much shorter shock structure with three peaks of wall heat flux similar to that of Mach number 2.2. However, as the inlet Mach number increased to 3.2, there is no shock structure upstream of fuel injections, and the combustor flow is in a supersonic mode with different locations and values of wall heat flux peaks. The statistical results of wall heat loading show that the change of total wall heat is not monotonic with the increase of inlet Mach number, and the maximum appears in the case of Mach number being 2.8. Meanwhile, for all the cases, the bottom wall takes up more than 50% of the total heat loading.

Experimental Study of Compact Multi-Pass Evaporator Two Phase Flow

The distribution of the liquid and gas flows in multi-pass channel that illustrate a compact evaporator for air-conditioning system was experimentally studies. The dividing header are design horizontal with a square cross section of 20mm x 20mm and length is 290mm, also ten curve multi-pass channel with length of 300mm each connected between the circle cross section 50mm combination tank. The distance between the dividing header and combing tank were 120mm. The test was conduct under uniform backpressure conditions; additionally, the working fluid used was water and air. This experiment conducted to examine the influence of inlet-flow at the header entrance (stratified flow and annular-mist flow) under uniform backpressure condition to improve the uniform of the water distribution inside every curve channels. It was found that the inlet-flow condition at the entrance header has relatively influence the water distribution. By comparison between two flow conditions, great value of uniformity for water distribution in each channel is improved under annular-mist flow condition.

Design of Spiral Casing of Francis Turbine for Micro Hydro Applications

Spiral casing is an important component of Francis turbine for even distribution of kinetic and potential energy of water so as to achieve the required reaction for better performance of the runner. Optimal design and analysis of spiral configuration is challenging, especially when the turbine is operating in a low head range due to the necessity of a large cross-sectional area of the casing to accumulate the large flow. The main objective of this study is to design the best configuration of cross section of spiral casing which have a very minimal pressure loss and can provide required inlet flow condition for stay vanes. For this, spiral casing configurations following free-vortex theory with different cross sections i.e. circular, trapezoidal and square are designed and analysed numerically. Pressure, radial and tangential velocity distributions are compared for different cross section of spiral casing suitable for micro hydro applications. Considering the ease of manufacturing, flexibility of dimension adjustments and comparable performance with that of the circular spiral casing, trapezoidal casing is found to be an appropriate alternative for such applications.

Hub clearance effects of a cantilevered tandem stator on the performance and flow behaviors in a small-scale axial flow compressor

The impacts of the increased hub clearance size (HCS) in the cantilevered tandem stator on the overall performance and flow behaviors have been investigated based on numerical simulations in a small-scale axial flow compressor stage. The results indicate that the HCS variation in the front blade of the tandem stator has a more significant effect on the compressor peak efficiency and stall margin. However, there is no obvious change of the compressor performance with the increase of the HCS in the rear blade. Detailed flow analysis shows that the increase of the HCS in the front blade has remarkable impacts on the flow fields in the front blade, while it has an only slight influence on the hub flow fields in the rear blade. It is found that the compressor efficiency penalty is due to the increase of the loss generation near the hub caused by the larger extent of hub leakage flow, while the stall margin improvement is due to the redistribution of the mass flow rate along the spanwise direction. As a result, the inlet flow condition within the upper blade span is improved and the flow blockage is reduced. The resulting influence of the increased HCS in the rear blade is mainly located near the blade leading edge of the rear blade, and there is no remarkable effect on the flow field in the front blade passage.

Investigation of Unsteady Pressure Fluctuations in Flow over Backward-Facing Step

Flow over a backward-facing step at , based on the step height, is simulated using the Divergence Free Synthetic Eddy Method of Poletto et al. (“A New Divergence Free Synthetic Eddy Method for the Reproduction of Inlet Flow Conditions for LES,” Flow, Turbulence and Combustion, Vol. 91, No. 3, 2013, pp. 519–539.), implemented in OpenFOAM® 1606, which preserves the instantaneous fluctuating pressure field in the domain, which is normally lost. Examination of the pressure spectra at various positions normal to the wall is well represented by wall-pressure measurements at all but the highest frequencies examined here. Various spectral peaks corresponding to shear-layer movement, discrete vortex shedding from the step, and numerous shear-layer instabilities are investigated using power and cross-spectral measurements of the fluctuating wall pressures. Convective velocities obtained from the cross-spectral phase indicate that the lowest frequencies convect slowly, whereas the higher frequencies near the measured spectral peaks convect at . There is an increase in convective velocity as the measurement locations are moved farther downstream. Additionally, an increase in peak frequency was found in this examination; however, the increase in convective velocity (approximately equal to 33%) and frequency (approximately equal to 7–11%) calculated at different streamwise positions were not found to be directly proportional, indicating that the increase in peak frequency cannot be solely explained by flow acceleration but that the structures are likely undergoing streamwise or spanwise deformation as well.

A novel procedure for validation of flow simulations in operating theaters

Heating, ventilation, and air conditioning (HVAC) systems must be specifically designed for operating theaters in hospitals in order to reduce the risk of infections and maintain adequate thermo-hygrometric conditions. New design procedures of operating theaters can benefit from the results obtained by properly validated computational fluid dynamic (CFD) models. In fact, CFD models allow obtaining local information inside the room without carrying out complex experiments, with consequent reduction of the costs required for testing ventilation systems. In this work, a novel procedure for validation of numerical simulations of airflow distribution in actual operating theaters is presented. The authors designed and carried out an experimental campaign in a new operating theater to measure airflow distribution near the surgical site and validate a novel numerical model with specific reference to the inlet flow conditions. The results prove that the present approach is essential to reproduce the actual flow conditions inside operating theatres and clearly shows the influence of flow boundary conditions on the numerical solution of air velocity in the room.

Two-phase gas-liquid flow in concentric and fully eccentric annuli. Part I: Flow patterns, holdup, slip ratio and pressure gradient

Horizontal and upward low-inclination gas-water and gas-oil flows are investigated using an annulus pipe configuration in a high-pressure system (∼400 kPa). The annulus test section, or the so-called pipe-in-pipe configuration, consists of an outer pipe of 99 mm inside diameter and an inner pipe of 50 mm outside diameter. Two different vertical positions of the inner pipe with respect to outer pipe have been tested; namely concentric configuration where both pipes have the same centreline and fully eccentric where the inner pipe is placed at the bottom wall of outer pipe. The flow is studied using high-speed photography, differential pressure transducers, and broad-beam gamma densitometers to characterise the flow as function of the liquid phase properties, annulus eccentricity, and pipe inclination. Flow regime maps reveal that slug flow appears to be the most common flow feature across the conditions studied in this paper. Flow in the eccentric annulus shows a more consistent behaviour (i.e. well-defined flow structures axially and in time) than that observed in the concentric annulus. Gas-oil flows show more stable phase fraction behaviour across the cross-section compared to that observed in gas-water flows in which the interface shows significant activity promoting the formation of droplets and ligaments that break from the liquid film. This is especially the case with the concentric annulus configuration. The oil phase regardless of the geometrical configuration tends to fully wet the pipe (made of PVC) as the gas velocity increases, creating a continuous film along the pipe walls. Conversely, the thin film in gas-water flow is not continuous. Discontinuity in the thin film increases with an increase in effective pipe roughness. Given the same inlet flow conditions, the concentric annulus configuration causes larger pressure drop along the pipe than that obtained in the eccentric.