THIN WALL LIQUID FILM THICKNESS MEASUREMENT: A VIDEO OPTICAL TECHNIQUE.

Transcription

1 THIN WALL LIQUID FILM THICKNESS MEASUREMENT: A VIDEO OPTICAL TECHNIQUE. S. Giroud-Garapon 1*, G. Heid 1, G. Lavergne 1, and O. Simonin 2 1 ONERA - Centre de Toulouse - DMAE, 2 avenue E. BELIN, BP Toulouse Cedex 4 France 2 Institut de Mécanique des Fluides de Toulouse, Allée du Professeur Camille Soula Toulouse France. KEYWORDS: Main subject(s): Optical Technique, Fluid: Multi phase flow, Visualization method(s): Video optical technique, Other keywords: Liquid film thickness, ABSTRACT : Wall spray interactions play an important role in the combustion efficiency prediction of turbojet or ramjet. They generate complex physical phenomena such as rebound onto wall or rebound onto wetted surface, splashing, deposition, film formation, film streaming and film atomization. ONERA/DMAE has been working on these subjects for few years, and some wall-drop interaction models have been developed and integrated into CFD-industrial-codes. In order to improve this work, a basic experimental study has been performed to analyze wall liquid film inside a combustion chamber. This is a cold flow experiment, where a liquid film is flowing on a hot tilted plate put on the bottom wall of the tunnel. Ethanol enriched with fluoresceine is used as fuel. The liquid emerges from a pipe with a diameter of 1 mm. Afterwards the film flow is canalized in a groove. It streams on the hot plate which temperature should be fixed from 300 K to700 K by an element heating controlled electronically. The film thickness is measured with a non-intrusive technique based on the laser trace displacement at the liquid film interface. Indeed, when the film thickness varies, the trace of the laser plan moves. Thus, it is enough to know the optical magnification used and the angle of the CCD camera to obtain the film thickness. This technique gives only the thickness of the film, so its velocity has to be estimated using flow rate conservation. The goal of the present experiment is to create an experimental data-base on wall liquid film behavior in terms of thickness, velocity and surface instabilities evolution (with an FFT analysis) for numerical comparison. 1. Introduction Combustion chamber development and their performance study are a field in which numerical simulations play an important role. The improvement of the predictions begins with the development of more and more sophisticated models, in order to take into account the maximum of physical phenomena. Nowadays, some of these phenomena associated with wall/spray interactions are always little or badly modelled and make that the drop size resulting from these impacts is not very well known. It is nevertheless this drop size which is important during the combustion chamber efficiency calculation. Numerical simulation improvements require the development of sophistical models. Some of them are already used in CFD codes, such as dispersion, combustion, secondary break-up (see Fig. 1). 1

2 FIG.1: PHYSICAL PHENOMENA IN A COMBUSTION CHAMBER. The purpose of this study is to pursue this effort by focusing on the streaming and the pulverization of a liquid film (see Fig.2). FIG 2: FILM STREAMING. In this way, a fine experimental study is organized. A film thickness measurement system was set up in order to establish an experimental data base for the validation of the numerical model which is developed in parallel [1]. 2. Experimental set up. A basic experimental study has been performed in order to analyze wall liquid film inside a combustion chamber (Giroud-Garapon and al. [1] & [2]). This is a cold flow experiment (see Fig. 3), where a liquid film is flowing on a flame holder put on the bottom wall of the tunnel. The experiment has a rectangular geometry with a cross section of 100*100 mm 2 with transparent walls. The air velocity is ranging from 30 to 100 m/s. FIG. 3: WIND TUNNEL. 2

3 Ethanol enriched with fluoresceine is used as fuel. The liquid injection is controlled by a flow metering unit. The liquid emerges from a pipe with a diameter of 1 mm (see Fig 4). Afterwards the film flow is canalized in a groove of 1mm depth and 10 mm width. It is streaming on the flame holder which temperature should be fixed from 300 K to 700 K by an element heating controlled electronically. FIG. 4: FILM STREAMING. 3. Film thickness measurement system. There are several techniques for wall liquid film thickness measurement. They are generally classified in two categories. The first one corresponds to the electric techniques which are often invasive and which generally require wall instrumentation (capacitance sensor Duckler et Bergelin [3], resistance Hewitt et al [4], ). The second one corresponds to the optical techniques (optical triangulation Decre [5], densitometry Cunha and Carbonaro [6], shadowgraphy Bosch [7], Moiré Schweizer [8], interferometry Hecht [9], ). They are non-invasive, but they require sometimes an instrumentation of the wall. The chosen technique must satisfy restrictive specifications. Indeed, it must be a non-intrusive technique in order to do not disturb the air flow and do not disturb the liquid flow. It mustn t require an instrumentation of the wall because of the temperature that can be met there (from K to K). Additionally, it must be a low cost technique. That s why the technique used is a video-optical technique based on laser trace displacement at the liquid film interface. The principle of this technique is based on the difference between the position of the laser trace without liquid film and with liquid film (see Fig. 5). A CCD camera gives us two pictures with and without film. The difference in pixel between these two positions can be obtained by image processing (see Fig. 7 to 10). Thus, it is enough to know the optical magnification and the camera angle used to obtain the film thickness. Nevertheless, in the reality the laser plan is not necessarily normal to the liquid film interface, so a corrective term must be introduced in order to take into account this angle. H r: H v H v Θ c Θ L G x Film thickness [m]. Film thickness seen by the camera [pixel]. Film thickness seen by the camera when Θ L =0 [pixel]. Camera angle. Laser angle with the normal. Optical magnification [m/pixel]. FIG.5: MEASUREMENT PRINCIPLE Thus: H r H v. Gx = Cos( θ ) c 1 1+ Tan( θ c ) * Tan( θ L ) corrective term due to laser angle 3

4 4. Validation. This technique has been validated on a basic experiment constituted by a jar and a capacitive sensor giving the height of liquid in the jar (see Fig. 6). Without liquid film. With liquid film. FIG. 6: VALIDATION OF THE FILM TECHNIQUE. A picture without film is used as reference, when the film thickness increases the laser trace position moves to the top of the picture (see Fig. 7 & 8). FIG.7: IMAGE WITHOUT FILM H r =0 µm. FIG.8: IMAGE WITH LIQUID FILM H r =950 µm. These images have been processed using a program (see Fig. 9 & 10). In this way, the laser trace outline can be analysed using a gradient method. FIG.9: IMAGE WITHOUT FILM H r =0 µm. FIG.10: IMAGE WITH LIQUID FILM H r =950 µm. The comparison between the capacitive technique and the selected method for this experiment show a good level of agreement especially if the different corrective terms are taken into account (see Fig. 11 & 12). Indeed, an error less than 6 per cent can be met on all the test range. 4

5 FIG. 11: COMPARAISON WITH SENSOR MEASURE FIG. 12: IMPORTANCE OF LASER AND CAMERA ANGLE. 5. Results. This technique has been integrated into the final experimental wind tunnel (Fig. 13). The videooptical assembly is fixed on a complex moving system that allows the film thickness measure all along the hot wall (from the injection point to the wall trailing edge). FIG. 13: TIME-DEPENDENT FILM THICKNESS SIGNAL. A typical result of the film thickness measurement as a function of time is presented in Fig. 14. It corresponds to an air velocity of V g =30m/s, a cold wall with T p =300K and a volume flux of D l =1,478 l/h. The measure is made at 12 mm from the injection point. The optical magnification used was 2.76 µm/pixel with a camera angle of Under the conditions given above the time mean film thickness is H=153 µm and there is no dominating wave frequency calculated using standard FFT routines. H[µm] t [s] FIG. 13: TIME DEPENDENT FILM THICKNESS SIGNAL. The analysis of the film thickness evolution considering different liquid flow rate shows that the film thickness logically decreases from the upstream to the downstream, which is normal considering 5

6 the air acceleration. On the other hand a more important flow logically leads to a thicker film (see Fig.14). FIG. 14: FILM THICKNESS EVOLUTION CONSIDERING DIFFERENT LIQUID FILM FLOW RATE. FIG. 15: FILM VELOCITY EVOLUTION CONSIDERING DIFFERENT LIQUID FILM FLOW RATE. The same remarks can be made concerning the velocity which has been estimated using flow rate conservation (see Fig.15). 6. Conclusion and prospects To avoid the cost and complexities of a full 3D computation, an integral film model is developed [1]. This model based on Foucart work [10] has to be validated with experimental results found in the literature as well as on those stemming from our experiment. This experiment which is beginning, allowed us to validate an innovating film thickness measurement technique. Actually everything is in place to develop an experimental data base allowing the validation of this numerical code. References [1] Giroud-Garapon, S., Heid, G., Lavergne G., Simonin O., Experimental and numerical study of thin wall liquid film spreading on a heating surface, ILASS Europe 2002, Saragossa, Spain. [2] Giroud-Garapon, S., Heid, Modélisation de la combustion diphasique dans les statoréacteurs, ONERA Laboratories Report No. 1/05604 DEFA/DMAE, [3] Duckler E.A. and Bergelin O.P., Characteristics of flow in falling liquid films, Chem Engng. Prog Vol , [4] Hewitt and al, Measurement of two phase flow parameters. Academic press Inc. LTD, [5] Decre et al, Contract report, VKI, [6] Cunha F. and Carbonaro M., the mechanics of thin film coating, ed. Gaskell P.H. et al, Singapore: World scientific. [7] Bosch, E.G.T., PhD Thesis, T.U. Eindhoven. [8] Schweizer P.M., Experimental methods in liquid film coating, S.F. Kistler & P.M. Schweizer eds, Chapman, London 1997, [9] Hecht E., Optics. Addison-Wesley [10] Foucart, H., Ph.D. Thesis, University of Rouen, France,