Faruk Al-Sibai

Experimental study of flow separation in laminar falling liquid films

In a previous publication, Dietze, Leefken & Kneer ( J. Fluid Mech. , vol. 595, 2008, p. 435) showed that flow separation takes place in the capillary wave region of falling liquid films. That investigation focused on the mechanistic explanation of the phenomenon mainly on the basis of numerical data. The present publication for the first time provides clear experimental evidence of the phenomenon obtained by way of highly resolving velocity measurements in a specifically designed optical test set-up. Characteristically, the refractive index of the working fluid was matched to that of the glass test section to provide optimal access to the cross-section of the film for the employed optical velocimetry techniques, namely, laser doppler velocimetry (LDV) and particle image velocimetry (PIV). Using LDV, time traces of the streamwise velocity component were recorded in high spatial (0.025 mm) and temporal resolutions (0.4 ms) showing negative velocity values in the capillary wave region. In addition, simultaneous film thickness measurements were performed using a Confocal Chromatic Imaging (CCI) technique enabling the correlation of velocity data and wave dynamics. Further, using PIV the spatio-temporal evolution of the velocity field in the cross-section of the film was measured with high spatial (0.02 mm) and temporal (0.5 ms) resolutions yielding insight into the topology of the flow. Most importantly these results clearly show the existence of a separation eddy in the capillary wave region. Due to the high temporal resolution of the PIV measurements, enabled by the use of a high-speed camera with a repetition rate of up to 4500 Hz, the effect of wave dynamics on the velocity field in all regions of the wavy film was elucidated. All experiments were performed using a dimethylsulfoxide (DMSO)–water solution and focused on laminar vertically falling liquid films with externally excited monochromatic surface waves. Systematic variations of both the Reynolds number ( Re = 8.6–15.0) and the excitation frequency ( f = 16–24 Hz) were performed. Results show that an increase in the wavelength of large wave humps, produced either by an increase in the Reynolds number or a decrease in the excitation frequency, leads to an increase in the size of the capillary separation eddy (CSE). Thereby, the CSE is shown to grow larger than the local film thickness, assuming an open shape with streamlines ending at the free surface.

Local and instantaneous distribution of heat transfer rates through wavy films

Abstract Heat transfer through a laminar-wavy falling silicon oil film on a vertical plate has been investigated. The film flows down an electrically heated metal foil which delivers a constant heat flux. The temperature field at the backside of the foil is measured by a very sensitive infrared camera with high temporal resolution. Local and instantaneous heat transfer rates through the film are evaluated from the temporal development of the local wall temperature. Investigations in two-dimensional waves show the influence of the Prandtl number on the transport processes even in the laminar region. The experimental data confirm the results of numerical calculations with a spectral element method. Furthermore, the temporal averaged heat transfer has been investigated in thermally developed three-dimensional wavy films over a film Reynolds number range of 10–129 and a Prandtl number range of 10–45. The Prandtl number dependency of the heat transfer in the laminar-wavy region of the film agrees well with a recently published experimental study.

Investigation of the thermal entry length in laminar wavy falling films

The use of liquid films flow offers solutions for the problems associated with the microgravity applications. The thermal entry length of laminar wavy falling films was experimentally determined under full gravity conditions by means of infrared thermography. A dependence of the entry length on the Reynolds, Prandtl, and Kapitza number as well as the ratio Pr0/PrW between the Prandtl numbers at inflow and wall temperatures was found.

Influence of Surface Roughness on Contact Heat Transfer

Almost all technical devices in use today are assemblies of individual pieces. For all force-based assembly methods, such as bolting or press-fitting, the thermal behavior is influenced by the contact resistance at the joint surface. For metal pieces in contact with each other, the authors have developed a measurement method and analysis tools enabling the determination of the contact heat transfer coefficient. Previously published results [1, 2, 3] have shown the dependence of the contact heat transfer coefficient on surface structure, contact pressure and material properties. The present work provides experimental and analytical data for the contact heat transfer coefficient and also proposes a model for calculating the real contact area of two surfaces which are placed under different contact pressures. Experiments were conducted for two material combinations with three different surface structures, while varying the contact pressures from 7 MPa to 230 MPa. When selecting average surface roughness (Rz ) as a characterizing parameter for surface structure, the results did not show a consistent trend. Thus, in this paper Rz was replaced by the real contact area between the two surfaces of interest. This area was determined by applying a refined method based on surface roughness measurements. The experimental data show a better consistency, when plotting the contact heat transfer coefficient relative to real contact area (Fk ) rather than the previously used Rz –values.Copyright

Measurement of instantaneous heat transfer using a hot-foil infrared technique

Hot-foil infrared thermography was employed to measure instantaneous local heat transfer coefficients of falling wavy films. The film flows down an electrically heated thin metal foil which provides a defined heat flux. The heat transfer coefficient of the film flow is evaluated from the temperature on the backside of the foil measured by an infrared camera. Good agreement of the instantaneous measurements with own numerical calculations points out the remarkable sensitivity and time resolution of the technique. The accuracy of the results is confirmed by comparison with recently published time averaged data.

Development of Heat Transfer in a Two-Dimensional Wavy Falling Film of Water and its Influence on Wave Stability

Heat exchangers employing falling films are relevant to a multitude of industrial applications using water-based liquids. In the present study, periodic, two-dimensional waves are imposed by excitation on a vertically falling film of water, which is then heated by a uniform heat flux, within the laminar and transitional flow range (39<Re<200). Liquid-film thickness is measured by confocal chromatic imaging and surface temperature is measured by high-speed IR thermography. As the 2D waves travel downstream they destabilize in the spanwise direction and evolve 3D structures (bumps). Further wave destabilization, under relatively low heating, was observed to coincide with the appearance of local thermal flows (“hot streaks”), though no deformation of the liquid surface could be measured. These flows are understood to be induced by thermo-capillary forces, which in extreme cases are known to lead to the formation of rivulets, film rupture and heater burnout. Understanding these initial stages of thermo-capillary flow is crucial to its suppression.Analysis of the thermal images reveals several significant streamwise length scales: a thermal inlet length based on the emergence of the thermal boundary layer (Lt), a thermal inlet length based on reaching thermally developed conditions (Lh), and the length at which “hot-streaks” first appear (Ls). In addition the dominant (most unstable) spanwise wavelength of the hot streaks, Lz, was identified through FFT analysis of the thermal profile beyond Ls. First the independence of the thermal inlet lengths from the heat-flux was established. Next, the influence of the nominal flow conditions (Reynolds number and excitation frequency) on Lt, Lh and Lz was examined — thereby extending the range of previous studies to higher Reynolds numbers. The thermal inlet lengths Lt and Lh were found to increase with flow rate, whereas they had opposing trends with regard to frequency. Lz consistently decreased with an increase of the flow rate, as smaller (turbulent) scales became more dominant, and it was found to be indifferent to excitation frequency over a wide range. Some future directions and methods of hot streak suppression are discussed, as well.© 2013 ASME

Evaluation of the sensitivity and response of IR thermography from a transparent heater under liquid jet impingement

The feasibility of a visible/IR transparent heater and its suitability for IR thermography is experimentally examined. The most common transparent conductive coating, Indium Tin Oxide (ITO), is quite reflective and its optical properties depend on thickness and manufacturing process. Therefore, the optical properties of several thicknesses and types of ITO, coated on an IR window (BaF2), are examined. A highly transparent Cadmium Oxide (CdO) coating on a ZnS window, also examined, is found to be unusable. Transmissivity is found to increase with a decrease in coating thickness, and total emittance is relatively low. A thick ITO coating was examined for IR thermography in the challenging test case of submerged water jet impingement, where temperature differences were characteristically small and distributed. The measurements under steady state conditions were found to agree well with the literature, and the method was validated. Comparison of two IR cameras did not show the LWIR low-temperature advantage, up to the maximal acquisition rate examined, 1.3KHz. Rather the MWIR camera had a stronger signal to noise ratio, due to the higher emissivity of the heater in this range. The transient response of the transparent heater showed no time-delay, though the substrate dampens the thermal response significantly. Therefore, only qualitative transient measurements are shown for the case of pulsating free-surface jet impingement, showing that the motion of the hydraulic jump coincides with thermal measurements. From these results, recommendations are made for coating/window combination in IR thermography.

Impinging Oil Jet Behaviour for Planar Wall Heat Transfer

Impinging jets for convective cooling are used in several technical applications. Piston cooling with impinging oil jets is one key application. To improve the heat transfer between the surface of the piston and the oil film it is necessary to understand the underlying mechanisms of heat transfer at the boundary face. For this reason it is important to analyze the oil flow and to identify and evaluate the influence of the parameters governing film formation. Also the oil jet is investigated, because the film formation can be influenced by the jet. In the experiments the oil temperature is set to 30 °C or 60 °C and the pressure at the nozzle inlet is varied between 1.6 bar and 4.2 bar. The minimal Reynolds number is 125 and the maximum is 1924. The liquid Weber number varies between 2.2 × 10−2 and 52.9 × 10−2 . The results of the visualization measurements reveal the influence of the exit velocity, oil temperature and the related material properties on the film formation process. On the one hand the results show the macroscopic relation between Reynolds number and the level of instability. On the other hand the relation between Weber number and the break-up at the surface of the jet and accordingly of the film surface can be demonstrated.Copyright