scispace - formally typeset
Search or ask a question

Showing papers on "Nozzle published in 2010"


Journal ArticleDOI
TL;DR: The transverse jet has been studied extensively because of its relevance to a wide variety of flows in technological systems, including fuel or dilution air injection in gas turbine engines, thrust vector control for high speed airbreathing and rocket vehicles, and exhaust plumes from power plants as discussed by the authors.

329 citations


Journal ArticleDOI
TL;DR: In this paper, the effects of the nozzle-exit conditions on the flow and sound fields of initially laminar jets are computed by large-eddy simulations (LES) to investigate the effects.
Abstract: Round jets originating from a pipe nozzle are computed by large-eddy simulations (LES) to investigate the effects of the nozzle-exit conditions on the flow and sound fields of initially laminar jets. The jets are at Mach number 0.9 and Reynolds number 105, and exhibit exit boundary layers characterized by Blasius velocity profiles, maximum root-mean-square (r.m.s.) axial velocity fluctuations between 0.2 and 1.9% of the jet velocity, and momentum thicknesses varying from 0.003 to 0.023 times the jet radius. The far-field noise is determined from the LES data on a cylindrical surface by solving the acoustic equations. Jets with a thinner boundary layer develop earlier but at a slower rate, yielding longer potential cores and lower centreline turbulent intensities. Adding random pressure disturbances of low magnitude in the nozzle also increases the potential core length and reduces peak r.m.s. radial velocity fluctuations in the shear layer. In all the jets, the shear-layer transition is dominated by vortex rolling-ups and pairings, which generate strong additional acoustic components, but also amplify the downstream-dominant low-frequency noise component when the exit boundary layer is thick. The introduction of inlet noise however results in weaker pairings, thus spectacularly reducing their contributions to the sound field. This high sensitivity to the initial conditions is in good agreement with experimental observations.

250 citations


Journal ArticleDOI
TL;DR: In this paper, the effects of the above parameters on grinding performance such as grinding forces and surface roughness have been investigated through experiment and modeling, and the results show that the setting location of the nozzle is an important factor regarding the effective application of MQL oil mist.
Abstract: Promising alternatives to conventional dry and fluid coolant applications are minimum quantity lubricant (MQL) or near dry grinding. Despite several researches, there have been a few investigations about the influence of MQL parameters on the process results, such as oil flow rate, air pressure, MQL nozzle position and distance from the wheel–workpiece contact zone. The current study aims to show through experiment and modeling, the effects of the above parameters on grinding performance such as grinding forces and surface roughness. The results show that the setting location of the nozzle is an important factor regarding the effective application of MQL oil mist. It has been shown that optimal grinding results can be obtained when the MQL nozzle is positioned angularly toward the wheel (at approximately 10–20° to the workpiece surface). In addition, it is found that the efficient transportation of oil droplets to the contact zone requires higher mass flow rate of the oil mist towards the grains flat area and longer deposition distance of an oil droplet. Applying the new setup, considerable reduction in the grinding forces and surface roughness has been achieved.

186 citations


Journal ArticleDOI
TL;DR: In this article, a two-dimensional model of the expansion of a collisionless, electron-magnetized, low-beta, current-free plasma in a divergent magnetic nozzle is presented.
Abstract: A two-dimensional model of the expansion of a collisionless, electron-magnetized, low-beta, current-free plasma in a divergent magnetic nozzle is presented. The plasma response is investigated in terms of the nozzle/plasma divergence rate, the magnetic strength on ions, and the Hall current at the nozzle throat. Axial acceleration profiles agree well with those estimated from simple one-dimensional models. A strong radial nonuniformity develops downstream. There is a separation between ion and electron/magnetic streamtubes which leads to the formation of, first, a longitudinal electric current density, which indicates that current ambipolarity is not fulfilled, and, second, a small ion azimuthal current that competes negatively with the electron azimuthal (Hall) current. The analysis of the mechanisms driving thrust, ion momentum, and ion energy unveils the dual electrothermal/electromagnetic character of the magnetic nozzle. In general, the thrust includes the contributions of volumetric and surface Hall currents, this last one formed at the plasma-vacuum interface. Plume efficiency, based on radial expansion losses, is computed. Plasma detachment and the transonic matching with the upstream plasma are not addressed.

180 citations


Journal ArticleDOI
01 Dec 2010-Fuel
TL;DR: In this paper, the authors used the mixture-based cavitation model in FLUENT v6.3 to characterize the inner nozzle flow dynamics of compression ignition engines and found that the inner-nail flow of both petrodiesel and biodiesel is significantly different.

161 citations


Journal ArticleDOI
TL;DR: In this paper, a mixture-based simulation of diesel injector cavitation and turbulence was performed using the FLUENT V6.2 software, and the results demonstrate the capability of the cavitation model to predict cavitating nozzle flows in realistic diesel injectors and provide boundary conditions, in terms of vapor fraction, velocity, and turbulence parameters at the nozzle exit, which can be coupled with the primary breakup simulation.
Abstract: Cavitation and turbulence inside a diesel injector play a critical role in primary spray breakup and development processes. The study of cavitation in realistic injectors is challenging, both theoretically and experimentally, since the associated two-phase flow field is turbulent and highly complex, characterized by large pressure gradients and small orifice geometries. We report herein a computational investigation of the internal nozzle flow and cavitation characteristics in a diesel injector. A mixture based model in FLUENT V6.2 software is employed for simulations. In addition, a new criterion for cavitation inception based on the total stress is implemented, and its effectiveness in predicting cavitation is evaluated. Results indicate that under realistic diesel engine conditions, cavitation patterns inside the orifice are influenced by the new cavitation criterion. Simulations are validated using the available two-phase nozzle flow data and the rate of injection measurements at various injection pressures (800-1600 bar) from the present study. The computational model is then used to characterize the effects of important injector parameters on the internal nozzle flow and cavitation behavior, as well as on flow properties at the nozzle exit. The parameters include injection pressure, needle lift position, and fuel type. The propensity of cavitation for different on-fleetmore » diesel fuels is compared with that for n-dodecane, a diesel fuel surrogate. Results indicate that the cavitation characteristics of n-dodecane are significantly different from those of the other three fuels investigated. The effect of needle movement on cavitation is investigated by performing simulations at different needle lift positions. Cavitation patterns are seen to shift dramatically as the needle lift position is changed during an injection event. The region of significant cavitation shifts from top of the orifice to bottom of the orifice as the needle position is changed from fully open (0.275 mm) to nearly closed (0.1 mm), and this behavior can be attributed to the effect of needle position on flow patterns upstream of the orifice. The results demonstrate the capability of the cavitation model to predict cavitating nozzle flows in realistic diesel injectors and provide boundary conditions, in terms of vapor fraction, velocity, and turbulence parameters at the nozzle exit, which can be coupled with the primary breakup simulation.« less

150 citations


Journal ArticleDOI
TL;DR: In this article, the effects of viscoelasticity on drop generation in inkjet printing were investigated via numerical simulations, using the Lagrangian-Eulerian finite element method.
Abstract: We investigate the effects of viscoelasticity on drop generation in inkjet printing. In drop-on-demand printing, individual ink ‘drops’ are ejected from a nozzle by imposed pressure pulses. Upon exiting the nozzle, the shape of each ‘drop’ is that of a nearly spherical bead with a long thin trailing ligament. This ligament subsequently breaks up under the Rayleigh instability, typically into several small droplets (known as satellite drops). These satellite drops can create unwanted splash on the target substrate and a reduction in printing quality. Satellite drops can potentially be eliminated by adding polymer to the ink; elastic stresses can act to contract the trailing ligament into the main drop before capillary breakup occurs. However, elasticity can also reduce the drop velocity and can delay or even prevent the break-off of the drop from the ink reservoir within the nozzle. To achieve optimal drop shape and speed, non-Newtonian parameters such as the polymer concentration and molecular weight must be chosen correctly. We explore this parameter space via numerical simulations, using the Lagrangian–Eulerian finite-element method of Harlen et al. (J Non-Newtonian Fluid Mech 60:81–104, 1995). Results are compared with experimental observations taken from real printheads.

131 citations


Journal ArticleDOI
01 Oct 2010-Fuel
TL;DR: In this paper, a special technique for visualizing the first 1.5 mm of the spray has been applied to examine the link between cavitation phenomenon inside the nozzle and spray behavior in the near-nozzle field.

123 citations


Journal ArticleDOI
01 Sep 2010-Fuel
TL;DR: In this article, the effects of fuel properties, temperature and pressure conditions on the extent of cavitation, flash boiling and, subsequently, primary break-up were investigated using real-size transparent nozzle of a gasoline injector from a modern direct-injection combustion system.

121 citations


Journal ArticleDOI
TL;DR: In this paper, a numerical model which simulates the powder flux distribution on a coaxial nozzle is presented, and a technique based on the selective weight measuring on planes perpendicular to the nozzle axis was used.

120 citations


Journal ArticleDOI
TL;DR: In this paper, an experimental and numerical study on laminar burning velocities of hydrogen-air mixtures was performed at standard pressure and room temperature varying the equivalence ratio from 0.8 to 3.0.

Journal ArticleDOI
TL;DR: In this article, the Homogenous Relaxation Model (HRM) is used to study thermal non-equilibrium, two-phase flows with flash-boiling and condensation.

Patent
Choong-Ho Lee1, Jung-Min Lee1
24 May 2010
TL;DR: In this article, a thin-film deposition apparatus is used to produce large substrates on a mass scale and improve manufacturing yield, and a barrier wall assembly including a plurality of barrier walls arranged in the first direction so as to partition a space between the first nozzle and the second nozzle.
Abstract: A thin film deposition apparatus used to produce large substrates on a mass scale and improve manufacturing yield. The thin film deposition apparatus includes a deposition source; a first nozzle disposed at a side of the deposition source and including a plurality of first slits arranged in a first direction; a second nozzle disposed opposite to the first nozzle and including a plurality of second slits arranged in the first direction; and a barrier wall assembly including a plurality of barrier walls arranged in the first direction so as to partition a space between the first nozzle and the second nozzle.

Journal ArticleDOI
TL;DR: In this article, the injection and atomization characteristics of biodiesel-ethanol blended fuel were investigated and compared with those from diesel and biodiesel fuels to obtain the atomisation characteristics of BNE.

Journal ArticleDOI
TL;DR: In this article, the effects of the nozzles number on the flow and power of cooling of a vortex tube are studied, using a three-dimensional numerical fluid dynamic model, and it is observed that as the number of nozzle is increased, power of the vortex tube increases significantly while cold outlet temperature decreases moderately.
Abstract: In this paper, the effects of the nozzles number on the flow and power of cooling of a vortex tube are studied, using a three-dimensional numerical fluid dynamic model. Vortex tube has interesting functions and several industrial applications, and, as a refrigerator, it is used as a spot cooling device in industry. High-pressure compressible fluid enters the vortex tube and leaves its cold and hot outlets at lower and higher temperatures than that of the inlet flow, respectively. A few experimental and a fewer numerical studies have been conducted in recent years on the effects of nozzles number of a vortex tube. However, in this paper, the effects of nozzles number on the flow and power of cooling of a counter-flow vortex tube are investigated comprehensively. It is observed that as the number of nozzles is increased, power of cooling increases significantly while cold outlet temperature decreases moderately.

Patent
11 Oct 2010
TL;DR: In this article, the authors present a combustor for combusting a flow of fuel and air, which consists of a number of fuel nozzles, a lean pre-nozzle fuel injection system, and a premixing annulus.
Abstract: The present application provides for a combustor for combusting a flow of fuel and a flow of air. The combustor may include a number of fuel nozzles, a lean pre-nozzle fuel injection system positioned upstream of the fuel nozzles, and a premixing annulus positioned between the fuel nozzles and the lean pre-nozzle fuel injection system to premix the flow of fuel and the flow of air.

Journal ArticleDOI
TL;DR: In this article, the authors investigated the effects of process parameters on the depth of flow separation and the dross attachment on the lower cut edge on the 10 mm stainless steel AISI 304 laser cut edge.
Abstract: The efficiency of the laser cutting process depends on both the rate of melting and rate of melt removal from the cut kerf. The depth of flow separation and the dross attachment on the lower cut edge relate to the efficiency of the melt removal process and can be used to characterize the rate of melt removal from the cut kerf. The melt flow velocity and melt film thickness are formulated in this study by consideration of the fundamentals of viscous incompressible fluid flow. The calculated melt flow velocity and melt film thickness are correlated with the depth of flow separation on the 10 mm stainless steel AISI 304 (EN 1.4301) laser cut edge. The effects of process parameters—including assist gas pressure, nozzle diameter, nozzle standoff, focal point position, and cutting speed—on the depth of flow separation and the dross attachment on the lower cut edge are investigated. The assist gas pressure, nozzle diameter, and focal point position are found to significantly affect the efficiency of melt removal from the cut kerf.

Journal ArticleDOI
M. S. Howe1
TL;DR: In this paper, an analysis of the noise generated during the passage of quiescent temperature/entropy inhomogeneities through regions of rapidly accelerated mean flow is made of the vortex sound generated by the jet.
Abstract: An analysis is made of the noise generated during the passage of quiescent temperature/entropy inhomogeneities through regions of rapidly accelerated mean flow. This is an important source of jet engine core noise. Bake et al. (J. Sound Vib., vol. 326, 2009, pp. 574–598) have used an ‘entropy wave generator’ coupled with a converging–diverging nozzle to perform a series of canonical measurements of the sound produced when the inhomogeneity consists of a nominally uniform slug of hot gas. When flow separation and jet formation occur in the diffuser section of the nozzle, it is shown in this paper that the vortex sound generated by the jet is strongly correlated with the entropy noise produced by the slug and that the overall noise level is significantly reduced. Streamwise ‘stretching’ of the hot slug during high subsonic acceleration into the nozzle and the consequent attenuation of the entropy gradient in the nozzle are shown to significantly decrease the effective rate at which indirect combustion noise increases with the Mach number. Numerical predictions indicate that this is responsible for the peak observed by Bake et al. in the entropy-generated sound pressure at a nozzle Mach number near 0.6.

Patent
07 Oct 2010
TL;DR: The inkjet recording apparatus includes an abnormal nozzle detective waveform signal generating device which generates a drive signal having abnormal nozzle detector waveform including a waveform that is different from a recording waveform and applied to pressure generating elements when performing ejection for abnormality detection to detect an abnormal one among nozzles.
Abstract: The inkjet recording apparatus includes: an abnormal nozzle detective waveform signal generating device which generates a drive signal having an abnormal nozzle detective waveform including a waveform that is different from a recording waveform and applied to pressure generating elements when performing ejection for abnormality detection to detect an abnormal nozzle among nozzles; an abnormal nozzle detective device which identifies the abnormal nozzle showing an ejection abnormality from results of the ejection for abnormality detection; a correction control device which corrects image data in such a manner that ejection is stopped from the identified abnormal nozzle and a desired image is recorded by the nozzles other than the abnormal nozzle; and a recording ejection control device which performs image recording by controlling ejection from the nozzles other than the abnormal nozzle in accordance with the image data that has been corrected by the correction control device

Journal ArticleDOI
TL;DR: The LIMMCAST facility as discussed by the authors was designed for modeling fluid flow and transport processes in the continuous casting of steel and operates at temperatures of 200-400°C by using the low melting point alloy SnBi.
Abstract: This paper presents the new experimental facility LIMMCAST which was designed for modeling fluid flow and transport processes in the continuous casting of steel. The facility operates at temperatures of 200–400°C by using the low melting point alloy SnBi. The main parameters of the facility, including the dimensions of the test sections, will be given. The resultant possibilities with respect to flow investigations in the tundish, in the submerged entry nozzle, and in the mould will be discussed. Over the period of assembling and commissioning the LIMMCAST facility, the small-scale set-up Mini-LIMMCAST was employed which uses the alloy GaInSn that is liquid at room temperatures. At this precursory facility an experimental program was started which is focused on quantitative flow measurements in the mould and in the submerged entry nozzle (SEN). The Ultrasound Doppler Velocimetry (UDV) and the Contactless Inductive Flow Tomography (CIFT) were applied to determine the flow structure within the mould. First experimental results will be presented here for a single and a two-phase flow in which argon gas bubbles were injected at the inlet of the SEN. According to the concept of the electromagnetic brake the impact of a DC magnetic field on the emergent jet flow from the SEN has been studied.

Patent
30 Mar 2010
TL;DR: A mixing chamber has a first inlet through which a first fluid is fed to the mixing chamber, and a second inlet extends and is operative to inject a second fluid into a stream of the first fluid as mentioned in this paper.
Abstract: A method and apparatus for mixing fluids, such as beverage syrup and water, uses countercurrent injection to improve blending of the fluids. A mixing chamber has a first inlet through which a first fluid is fed to the mixing chamber, and a second inlet through which a countercurrent injection nozzle extends and is operative to inject a second fluid into a stream of the first fluid. The countercurrent injection nozzle is equipped with a check valve to control the flow of fluid into the mixing chamber. The mixing chamber may include additional inlets that may be fitted with countercurrent injection nozzles to permit the countercurrent injection of other fluid, such as flavorings, into the stream of the first fluid.

Patent
06 Apr 2010
TL;DR: In this article, a combustor section is provided and includes one or more annular quaternary fuel manifolds mounted within an annular passage defined between a casing and a cap assembly of a combustionor through which air and/or a fuel/air mixture flows upstream from a fuel nozzle support.
Abstract: A combustor section is provided and includes one or more annular quaternary fuel manifolds mounted within an annular passage defined between a casing and a cap assembly of a combustor through which air and/or a fuel/air mixture flows upstream from a fuel nozzle support, the manifold including a body to accommodate quaternary fuel therein, the body defining injection holes through which the quaternary fuel is injected into a section of the passage at a location upstream from the fuel nozzle support.

Patent
21 Jul 2010
TL;DR: In this paper, the authors describe a body having a base portion opposite a nozzle portion and a fuel passageway extending from the base portion to the nozzle portion, where the first valve is movable in response to actuation from the force generator to move between closed and open positions.
Abstract: The present disclosure is directed to integrated injector/igniters providing efficient injection, ignition, and complete combustion of various types of fuels. One example of such an injectors/igniter can include a body having a base portion opposite a nozzle portion, and a fuel passageway extending from the base portion to the nozzle portion. A force generator and a first valve are carried by the base portion. The first valve is movable in response to actuation from the force generator to move between closed and open positions. The injector/igniter also includes a second valve at the nozzle portion that is deformable in response to pressure in the fuel passageway to deform between a closed position and an open position.

Journal ArticleDOI
TL;DR: It is demonstrated that new nozzle designs that eliminate emulsion preparation prior to spray drying can be beneficial for microencapsulation applications, however, there is still a need for research to improve micro Encapsulation efficiency of multiple channel spray nozzles.
Abstract: Microencapsulation improves oxidative stability and shelf life of fish oil. Spray and freeze drying are widely used to produce microcapsules. Newer spray-nozzles utilize multiple fluid channels allowing for mixing of wall and core materials at the point of atomization. Sonic energy has also been employed as a means of atomization. The objective of this study was to examine the effect of nozzle type and design on fish oil encapsulation efficiency and microcapsule properties. A total of 3 nozzle types, a pressure nozzle with 1 liquid channel, a pressure nozzle with 2 liquid channels, and a sonic atomizer with 2 liquid channels were examined for their suitability to encapsulate fish oil in whey protein isolate. Physical and chemical properties of freeze dried microcapsules were compared to those of microcapsules produced by spray drying. The 2-fluid pressure and ultrasonic nozzles had the highest (91.6%) and the lowest microencapsulation efficiencies (76%), respectively. There was no significant difference in bulk density of microcapsules produced by ultrasonic and 3-fluid pressure nozzles. The ultrasonic nozzle showed a significantly narrower particle size distribution than the other nozzles. This study demonstrated that new nozzle designs that eliminate emulsion preparation prior to spray drying can be beneficial for microencapsulation applications. However, there is still a need for research to improve microencapsulation efficiency of multiple channel spray nozzles. Practical Application: Since this research evaluates new spray nozzle designs for oil microencapsulation, the information presented in this article could be an interest to fish oil producers and food industry.

Patent
16 Dec 2010
TL;DR: In this paper, a powered irrigator for use in rinsing nasal cavities has a main body including a handle and a fluid reservoir detachably coupled to one another, an outlet nozzle extending from a top end of the handle, and a switch operably coupled with the power source for turning the pump mechanism on and off.
Abstract: A powered irrigator for use in rinsing nasal cavities has a main body including a handle and a fluid reservoir detachably coupled to one another, an outlet nozzle extending from a top end of the handle, a pump mechanism operably coupled to a power source, and a switch operably coupled to the power source for turning the pump mechanism on and off. When the switch turns on the pump mechanism, fluid flows from the fluid reservoir into a first fluid coupling between the reservoir and the pump mechanism and into a second fluid coupling between the pump mechanism to the outlet nozzle. The top of the handle can be angled relative to a longitudinal axis of the main body. The nozzle is removable from the top end and has a self-sealing structure to provide an enhanced fit within a user's nostril.

Patent
10 Jun 2010
TL;DR: In this article, a fuel injection valve with a simple structure and easily changing fuel injection characteristics is proposed to ensure stable operation of the fuel injector valve, reducing a variation among products by simple structure.
Abstract: PROBLEM TO BE SOLVED: To provide a fuel injection valve ensuring stable operation of the fuel injection valve, reducing a variation among products by a simple structure, and easily changing fuel injection characteristics. SOLUTION: This fuel injection valve includes: a body 1; an orifice plate 2 provided under the body 1; a command piston guide 6 provided under the orifice plate 2; a command piston 5 internally provided to the orifice plate 2 and command piston guide 6; a spring receiver 3 internally provided to the command piston guide 6; a spring 4 downwardly pressing the spring receiver 3; a nozzle body 8 arranged below the command piston guide 6; and a nozzle needle 7 internally provided to the nozzle body 8. The command piston 5 abuts on the lower surface of the body 1, thereby limiting the lift of the nozzle needle 7. COPYRIGHT: (C)2010,JPO&INPIT

Journal ArticleDOI
TL;DR: It is shown that in addition, a high-speed air jet is pushed out of the cavity, which closely resembles that of compressible flow through a nozzle-with the key difference that the "nozzle" is a liquid cavity shrinking rapidly in time.
Abstract: A solid object impacting on liquid creates a liquid jet due to the collapse of the impact cavity. Using visualization experiments with smoke particles and multiscale simulations, we show that in addition, a high-speed air jet is pushed out of the cavity. Despite an impact velocity of only 1 m/s, this air jet attains supersonic speeds already when the cavity is slightly larger than 1 mm in diameter. The structure of the air flow closely resembles that of compressible flow through a nozzle—with the key difference that here the “nozzle” is a liquid cavity shrinking rapidly in time

Journal ArticleDOI
05 Jan 2010-Langmuir
TL;DR: The breakup of dense aggregates in an extensional flow was investigated experimentally and it was found, that neither aggregate structure, characterized by a fractal dimension d(f) = 2.7, nor shape, defined by an average aspect ratio equal to 1.5, was affected by breakage.
Abstract: The breakup of dense aggregates in an extensional flow was investigated experimentally. The flow was realized by pumping the suspension containing the aggregates through a contracting nozzle. Variation of the cluster mass distribution during the breakage process was measured by small-angle light scattering. Because of the large size of primary particles and the dense aggregate structure image analysis was used to determine the shape and structure of the produced fragments. It was found, that neither aggregate structure, characterized by a fractal dimension df = 2.7, nor shape, characterized by an average aspect ratio equal to 1.5, was affected by breakage. Several passes through the nozzle were required to reach the steady state. This is explained by the radial variation of the hydrodynamic stresses at the nozzle entrance, characterized through computational fluid dynamics, which implies that only the fraction of aggregates whose strength is smaller than the local hydrodynamic stress is broken during one ...

Journal ArticleDOI
TL;DR: In this article, a closed-loop impingement spray cooling system to cool a 1kW 6U electronic test card has been conducted, which uses R134a as working fluid in a modified refrigeration cycle.

Journal ArticleDOI
TL;DR: In this article, the effects of spray height, nozzle spray angle, inlet pressure and spray incident angle on heat transfer of spray cooling were studied by an experimental method, where Doppler anemometry and a camera were applied to study the spray flow field.
Abstract: The effects of spray height, nozzle spray angle, inlet pressure and spray incident angle on heat transfer of spray cooling were studied by an experimental method. Multi-points thermocouples and infrared imaging device were used to measure temperature distribution on heating surface. A Doppler anemometry and a camera were applied to study the spray flow field. The mechanism of heat transfer of spray cooling was concluded on the basis of experimental data and spray characteristics. It is showed that parameters affect heat transfer by way of changing the flow field on the heating surface. Heat transfer performance can be optimized by a smaller spray angle nozzle, which sprays at a smaller spray height and a higher inlet pressure. The effect of incident angle on heat transfer depends on nozzle spray angle and the definition of distance of nozzle to surface.