Flow characteristics of spray impingement in PFI injection systems

The present paper addresses an experimental study of the dynamic exchanges between the impact of an intermittent spray and the liquid film formed over the target, based on detailed phase Doppler anemometry (PDA) measurements of droplet size, velocity and volume flux in the vicinity of the impact. The flow configuration is that of a pulsed injector spraying gasoline onto a flat disc to simulate the port fuel injection (PFI) of an internal combustion engine operating at cold-start conditions. The measurements evidence that the outcome of impact cannot be accurately predicted based on the characteristics of the free spray, but requires precise knowledge of the flow structure, induced by the target. The implications for spray–wall interaction modelling are then discussed based on the application of conservation equations to the mass, momentum and energy exchanged between the impinging droplets and the liquid film. The results show that the liquid film starts to form in the vicinity of the stagnation region at early stages of injection and a non-negligible proportion of droplets impinging at outer regions splash after interaction with the film. Film disruption is mainly driven by the intermittent axial momentum of impinging droplets, which enhances the vertical oscillations. The radial momentum imparted to the liquid film at the stagnation region is fed back onto secondary droplets emerging later during the injection cycle at outwards locations, where momentum of impacting droplets is much smaller. As a consequence, although the number of splashed droplets is enhanced by normal momentum, their size and ejection velocity depends more on the radial spread induced onto the liquid film and, hence, on the radial momentum at impact. The analysis further shows that existing spray–wall interaction models can be improved if the dynamic exchanges between the impacting spray and the liquid film are accounted.     

Two-phase structure above hot surfaces in jet impingement boiling

Jet impingement boiling is very efficient in cooling of hot surfaces as a part of the impinging liquid evaporates. Several studies have been carried out to measure and correlate the heat transfer to impinging jets as a function of global parameters such as jet subcooling, jet velocity, nozzle size and distance to the surface, etc. If physically based mechanistic models are to be developed, studies on the fundamentals of two-phase dynamics near the hot surface are required. In the present study the vapor–liquid structures underneath a subcooled (20 K) planar (1 mm × 9 mm) water jet, impinging the heated plate vertically with a velocity of 0.4 m/s, were analyzed by means of a miniaturized optical probe. It has a tip diameter of app. 1.5 μm and is moved toward the plate by a micrometer device. The temperature controlled experimental technique enabled steady-state experiments in all boiling regimes. The optical probe data provides information about the void fraction, the contact frequencies and the distribution of the vapor and liquid contact times as a function of the distance to the surface. The measured contact frequencies range from 40 Hz at the onset of nucleate boiling to nearly 20,000 Hz at the end of the transition boiling regime. Due to condensation in the subcooled jet vapor disappears at a distance to the surface of app. 1.2 mm in nucleate boiling. This vapor layer becomes smaller with increasing wall superheat. In film boiling a vapor film thickness of 8 ± 2 μm was found.     

Simultaneous droplet impingement dynamics and heat transfer on nano-structured surfaces

This study examines the hydrodynamics and temperature characteristics of distilled deionized water droplets impinging on smooth and nano-structured surfaces using high speed (HS) and infrared (IR) imaging at We = 23.6 and Re = 1593, both based on initial drop impingement parameters. Results for a smooth and nano-structured surface for a range of surface temperatures are compared. Droplet impact velocity, transient spreading diameter and dynamic contact angle are measured. The near surface average droplet fluid temperatures are evaluated for conditions of evaporative cooling and boiling. Also included are surface temperature results using a gold layered IR opaque surface on silicon. Four stages of the impingement process are identified: impact, boiling, near constant surface diameter evaporation, and final dry-out. For the boiling conditions there is initial nucleation followed by severe boiling, then near constant diameter evaporation resulting in shrinking of the droplet height. When a critical contact angle is reached during evaporation the droplet rapidly retracts to a smaller diameter reducing the contact area with the surface. This continues as a sequence of retractions until final dry out. The basic trends are the same for all surfaces, but the nano-structured surface has a lower dissipated energy during impact and enhances the heat transfer for evaporative cooling with a 20% shorter time to achieve final dry out.     

Local heat transfer from a hot plate to a water jet

Jet impingement boiling is very efficient in cooling of hot surfaces as a part of the impinging liquid evaporates. Because of its importance to many cooling procedures, investigations on basic mechanisms of jet impingement boiling heat transfer are needed. Until now, most of the experimental studies, carried out under steady-state conditions, used a heat flux controlled system and were limited by the critical heat flux (CHF). The present study focuses on steady-state experiments along the entire boiling curve for hot plate temperatures of up to 700°C. A test section has been built up simulating a hot plate. It is divided into 8 independently heated modules of 10 mm length to enable local heat transfer measurements. By means of temperature controlled systems for each module local steady-state experiments in the whole range between single phase heat transfer and film boiling are possible. By solving the two dimensional inverse heat conduction problem, the local heat flux and the corresponding wall temperature on the surface of each module can be computed. The measurements show important differences between boiling curves measured at the stagnation line and those obtained in the parallel flow region. At the stagnation line, the transition boiling regime is characterised by very high heat fluxes, extended to large wall superheats. Inversely, boiling curves in the parallel flow region are very near to classical ones obtained for forced convection boiling. The analysis of temperature fluctuations measured at a depth of 0.8 mm from the boiling surface enables some conclusions on the boiling mechanism in the different boiling regimes.     

Hollow droplets impacting onto a solid surface

The impact process of spherical hollow droplets impinging onto a solid surface has been experimentally studied. Formation of a counter-jet in a wide range of Reynolds and Weber numbers was revealed, this jet being similar to a Worthington jet. For characterizing the regime of liquid flow in the hollow droplet, we propose using the Euler number. Theoretically, the problem was treated using a simple model of axisymmetric liquid flow. The obtained results proved to be consistent with experimental data.     

Compositive effects of orientation and contact angle on critical heat flux in pool boiling of water

An experimental study was carried out to investigate the effects of heat transfer surface orientation and the solid–liquid contact angle on the boiling heat transfer and critical heat flux (CHF) in water pool boiling using a smooth heat-transfer surface under atmospheric pressure. The orientation angle was ranged from 0° (up-facing horizontal position) to 180° (down-facing horizontal position) with a pace of 45°. The three kinds of heat transfer surfaces having different solid–liquid contact angles were the normal surface with a contact angle of 55°, the hydrophilic surface with a contact angle of 30° and the superhydrophilic surface with a contact angle of 0°. The experimental results indicate that orientation and contact angle have complex, coupling effects on heat transfer and CHF. A predicting correlation for the CHF which takes the effects of both orientation and contact angle into account is established. The predicting correlation agrees reasonably well with the experimental data.     

Computational study of bouncing and non-bouncing droplets impacting on superhydrophobic surfaces

We numerically investigate bouncing and non-bouncing of droplets during isothermal impact on superhydrophobic surfaces. An in-house, experimentally validated, finite element method-based computational model is employed to simulate the droplet impact dynamics and transient fluid flow within the droplet. The liquid–gas interface is tracked accurately in Lagrangian framework with dynamic wetting boundary condition at three-phase contact line. The interplay of kinetic, surface and gravitational energies is investigated via systematic variation of impact velocity and equilibrium contact angle. The numerical simulations demonstrate that the droplet bounces off the surface if the total droplet energy at the instance of maximum recoiling exceeds the initial surface and gravitational energy, otherwise not. The non-bouncing droplet is characterized by the oscillations on the free surface due to competition between the kinetic and surface energy. The droplet dimensions and shapes obtained at different times by the simulations are compared with the respective measurements available in the literature. Comparisons show good agreement of numerical data with measurements, and the computational model is able to reconstruct the bouncing and non-bouncing of the droplet as seen in the measurements. The simulated internal flow helps to understand the impact dynamics as well as the interplay of the associated energies during the bouncing and non-bouncing. A regime map is proposed to predict the bouncing and non-bouncing on a superhydrophobic surface with an equilibrium contact angle of 155°, using data of 86 simulations and the measurements available in the literature. We discuss the validity of the computational model for the wetting transition from Cassie to Wenzel state on micro- and nanostructured superhydrophobic surfaces. We demonstrate that the numerical simulation can serve as an important tool to quantify the internal flow, if the simulated droplet shapes match the respective measurements utilizing high-speed photography.     

The coupling influence of airflow and temperature on the wall-wetted fuel film distribution

The coupling influence of airflow and temperature on the two-dimensional distribution of the film resulted from fuel spray impinging on a horizontal flat wall was studied with experiments. The horizontal airflow direction was perpendicular to the vertical axis of the injection spray. The results show that, as air velocity increases, the film shape turns from a circle to an oblong. As wall temperature increases, the film area shrinks. Film thickness decreases as wall temperature or air velocity increases. The boiling point of the fuel is an important temperature to affect the film area and the film thickness. Film center moves more far away in the downstream direction as air velocity increases. For a certain air velocity, film center moves less far away as wall temperature increases.     

A film boiling model for cryogenic chilldown at low mass flux inside a horizontal pipeline

A film boiling heat transfer model is developed for cryogenic chilldown at low mass flux inside a horizontal pipeline. It incorporates the stratified flow structure and is based on conservation principles of mass, momentum, and energy. Simplifying assumptions lead to an expression for the local film boiling heat transfer coefficient which varies with the azimuthal angle. The efficacy of the model is assessed by comparing the predicted wall temperature histories with those measured at several azimuthal positions and various mass fluxes. Good agreement is observed at low flux, G = 13–54 kg/m² s.     

Numerical simulation of flow boiling for organic fluid with high saturation temperature in vertical porous coated tube

A semi-analytical model is developed for the prediction of flow boiling heat transfer inside vertical porous coated tubes. The model assumes that the forced convection and nucleate boiling coexist together in the annular flow regime. Conservations of mass, momentum, and energy are used to solve for the liquid film thickness and temperature. The heat flux due to nucleate boiling consists of those inside and outside micro-tunnels. To close the equations, a detailed analysis of various forces acting on the bubble is presented to predict its mean departure diameter. The active nucleation site density of porous layer is determined from the pool boiling correlation by introducing suppression factor. The flow boiling heat transfer coefficients of organic fluid (cumene) with high saturation temperature in a vertical flame-spraying porous coated tube are studied numerically. It is shown that the present model can predict most of the experimental values within ±20%. The numerical results also indicate that the nucleate boiling contribution to the overall heat transfer coefficient decreases from 50% to 15% with vapor quality increasing from 0.1 to 0.5.     

Experimental study of water droplet boiling on hot, non-porous surfaces

In this paper, the results of a series of experimental tests on single- and multi-droplet boiling systems are presented and discussed. The main objectives of the present study are: a) to investigate experimentally the effect of the boiling onset on the evaporation rate of water droplets; b) to measure the evolution of the solid surface temperature during evaporation; c) to examine the possibility of improving spray cooling efficiencies. The behavior of small water droplets (from 10 to 50 μl) gently deposited on hot, non-porous surfaces is observed. The evaporation of multi-droplet arrays (50 and 100 μl) under the same conditions of the single-droplet tests is analyzed. In particular, the conditions which determine the onset of nucleate and film boiling are stressed out. In the experimental tests, the interaction of different materials with several multi-droplet systems is monitored by infrared thermography. The spray cooling efficiency is related to the solid temperature decrease as a function of the water mass flux. In the present study, the effect of varying the droplet volume and the mass flux is also analyzed and discussed. The results on the droplets evaporation time and on the solid surface transient temperature distribution are also compared with the data obtained by the same authors during the analysis of droplet evaporation in total absence of nucleate and film boiling. In order to analyze the different behavior of the evaporating droplet as a function of the solid surface thermal conductivity, evaporative transients on aluminum, stainless steel and macor (a glass-like, low-conductivity material) are considered. Received on 20 February 1998     

Calculating hydrodynamics problems for a liquid film flowing radially on a flat horizontal surface

A mathematical hydrodynamic model of a thin liquid film flowing radially on a flat horizontal surface has been elaborated. The model consisted of continuity and momentum equations which were resolved by means of the integral method. In fact, several versions of the model were examined; they differed mainly in film velocity distribution. The predictions of the different versions were then compared with liquid film thicknesses obtained from experimental investigation. The best version was applied in further calculations.     

Boiling heat transfer region with independence of the wall temperature

 Extensive measurements of the intensive cooling of hot-rolled wires with temperatures between 1000 ^°C and 1100 ^°C are analysed. The analysis proves the existence of a convection-controlled boiling region, which has been previously observed by few authors in the case of high mass fluxes and high liquid subcooling. This region is characterised by an independence of the heat flux of the surface temperature. The heat flux depends essentially on the Reynolds number, the main influence parameter of the single phase convection, and on the liquid subcooling. Received on 13 September 1999     

The effect of surface condition and rotation on film / transition boiling from a sphere

The effects of liquid subcooling and surface condition on transition boiling heat transfer from a rotating surface were investigated in a series of experiments in which Freon 113 was boiled on a spinning sphere. In addition to the usual thermal measurements, the boiling behavior in the transition regime was studied by visual means. It was found that the shape of the system's boiling curve was very dependent on the parameters in question. Hydrodynamic instabilities created in the film boiling regime by the rotation of the sphere were carried on to the transition and nucleate boiling regimes of the boiling curve. Boiling curves exhibiting more than one peak flux were obtained.     

Experimental study of the flow regimes resulting from the impact of an intermittent gasoline spray

The present paper reports a complete set of measurements made with a two-component phase Doppler anemometer of the two-phase flow generated at the impact of a transient gasoline spray onto a flat surface. The spray is generated by a pintle injector and the fuel used was gasoline. The measurements of droplet size–velocity were processed to provide time fluxes of number, mass, normal momentum, and energy of the poly-dispersion of droplets ejected at impact, and analyzed based on predictive tools available in the literature. The results show that splash is the dominant mechanism by which secondary droplets are ejected from the surface, either in the stagnation region or in the core region of the spray. In the stagnation region, a large fraction of each incident droplet adheres to the surface and the axial incident momentum contributes with a larger parcel than tangential momentum. As a result, the normal velocity of ejected droplets is much smaller than that of the original incident droplets, while tangential velocity is enhanced. The region near the stagnation point is immediately flooded upon impact of the leading front of the spray, forming a liquid film that is forced to move radially outwards as droplets continue to impinge during the steady period. Spray/wall interaction in the core region thus occurs in the presence of a moving thin liquid film, which enhances transfer of tangential momentum. As a result, film spreading and dynamics as a result of impingement forces are crucial to accurate model spray/wall interaction. The outer region of the spray is dominated by the vortical structure induced by shear forces, which entrains small responsive secondary droplets to re-impinge. Furthermore, prediction of the outcome of spray impact requires a precise knowledge of the two-phase flow in the presence of the target.     

Nonlinear vibrations of a vapor film in the presence of intense heat fluxes

Waves propagating along the interface between a thin vapor film and a liquid layer in the presence of a heat flux are investigated. The boundary conditions on the vapor-liquid phase surface take into account the temperature dependence of the pressure and the possibilities of formation of the metastable state of the superheated liquid and mass flow. Variations in the saturation pressure as functions of the temperature and mass flux lead to generation of weakly damped periodic waves of low amplitude whose velocity can be much higher than the velocity of the gravity waves. The waves ensure stability of the vapor film beneath the liquid layer in the gravity field. The finite-amplitude waves on the surface of the vapor film differ from the Stokes surface waves on the free surface of isothermal fluid. Instability regimes related with superheating of the liquid ant its explosive boiling when the amplitude of an initially small wave increases to infinity in a finite time can develop in a certain working-parameter regime.     

Single droplet impingement of urea water solution on a heated substrate

In the present work, impingement behaviour of an aqueous urea solution is investigated experimentally. The effects of droplet diameter, impact velocity and substrate temperature are evaluated by monitoring single droplet impingement with a high-speed camera. Results allow the formulation of four different interaction regimes and a regime map depending on hydrodynamic and thermal parameters is proposed. The regimes deposition, splash, boiling-induced breakup, rebound with breakup and the transition boundaries are discussed in detail. Results show that the solute significantly affects the outcome of droplet impingement promoting droplet disintegration by enhanced nucleation and bubble formation. Comparison with literature data reveal the strong dependency of droplet impact behavior on the Weber number as a combination of initial droplet diameter and impact velocity.