Observing the thermodynamic effects in cavitating flow by IR thermography

When dealing with liquid flows, where operating temperature gets close to the liquid critical temperature, cavitation cannot be assumed as an isothermal phenomenon. Due to the relatively high density of vapor, the thermodynamic effect (decrease of temperature in the bulk liquid due to latent heat flow) becomes considerable and should not be neglected. For applications like pumping cryogenic fuel and oxidizer in liquid propulsion space launchers, consideration of the thermodynamic effect is essential - consequently the physical understanding of the phenomenon and its direct experimental observation has a great value. This study presents temperature measurements in a cavitating flow on a simple convergent-divergent constriction by infrared (IR) thermography. Developed cavitating flow of hot water (-100 °C) was evaluated by high-speed IR thermography and compared with conventional high-speed visualization, at different operating conditions with the velocity range at the nozzle throat between 9.6 and 20.6 m/s and inlet pressure range between 143 and 263 kPa. Temperature depression near the nozzle throat - near the leading edge of cavitation was measured in a range up to [vartriangle]T = 0.5 K. This confirms the presence of the thermodynamic effects by cavitation phenomenon and it is in agreement with its theory. In the study, average temperature fields, fields of temperature standard deviation and time-resolved temperatures, are presented and discussed. In addition, statistical analysis between temperature drop and cavitation flow characteristics is shown

Cavitation dynamics in water at elevated temperatures and in liquid nitrogen at an ultrasonic horn tip

Understanding and predicting thermodynamic effects is crucial when the critical point temperature is close to the operating temperature of the fluid, like in cryogenics. Due to the extreme difficulties of experimental investigation, predicting of thermodynamic effects in cavitation often bases on data in liquids other than cryogenics. Most often used surrogate liquids are hot water or certain refrigerants, which are selected by a single fluid property, most commonly by the thermodynamic parameter ∑. The paper presents a systematic study of the cavitation dynamics in water at 20 °C, 40 °C, 60 °C, 80 °C and 100 °C and in addition in liquid nitrogen (LN2). Cavitation dynamics on a 4.8 mm (tip diameter) ultrasonic horn tip, which oscillated at 20 kHz was investigated by high-speed visualization at 300,000 frames per second (fps). Simultaneously acoustic emissions were recorded by a high frequency pressure transducer. Measurements were performed under variation of the acoustic power in a closed, insulated vessel, where pressure could be optionally set. The main purpose of the presented investigation is to determine whether hot water can act as a surrogate liquid to cryogenics. The results may implicate the future investigations and development of a new generation of rocket engines, which also feature the possibility of re-ignition while in orbit – understanding and predicting of cavitation behaviour is becoming a crucial part at the (liquid oxygen – LOX and liquid hydrogen – LH2) turbo-pump design

Thermodynamic effects in cavitating flow

Cavitation is a phenomenon characterized by vapor generation and condensation in liquid flows, at approximately constant temperature. Cavitation is usually triggered by a local pressure drop in the vicinity of a cavitation nucleus. As the bubble grows, latent heat is supplied from the surrounding liquid to the interface, creating a thermal boundary layer. This creates a thermal boundary layer. The result is a local drop in the liquid temperature, which leads to a drop in vapor pressure. This delays the further development of the bubble, as a greater pressure drop is required to maintain the process. This phenomenon is known as thermal delay or thermodynamic effect and plays a moderating role in the development of cavitation. Thermodynamic effects can usually be neglected for liquids whose critical temperature is much higher than the operating temperature. However, this is not the case for liquids whose operating temperature is close to the critical temperature, such as in cryogenics. The understanding of the thermodynamic effects of cavitating flow is crucial for applications like turbopumps for liquid hydrogen LH2 and oxygen LOx in space launcher engines. Experimental studies of this phenomenon are rare as most of them were performed in the 1960s and 1970s. Due to the extreme difficulties of experimental investigation, predicting of thermodynamic effects in cavitation often bases on data in liquids other than cryogenics. Most often used surrogate liquids are hot water or certain refrigerants, which are selected by a single fluid property, most commonly by the thermodynamic parameter ∑

Thermo-mechanical modeling of polymer spur gears with experimental validation using high-speed infrared thermography

The presented work is focused on the development of a comprehensive thermo-mechanical model for predicting the temperature rise in thermoplastic polymer spur gears with any desired profile geometry while running. The specific constitutional behavior of thermoplastics influences the gear-meshing pattern, which can deviate substantially from ideal gear meshing, as typically exhibited by metal gears in moderate-loading conditions. Taking this aspect into account is of paramount importance if realistic temperature-rise predictions are to be made. The thermal response of the considered gear pair is studied thoroughly from both the analytical and experimental standpoints. Good agreement was found between the results of the model and the experimental measurements performed using a high-speed thermal imaging infrared camera, although it was also observed that the real-life temperature rise can increase noticeably if large geometric tolerance deviations from the ideal profile geometry are present. The presented experimental approach also offers the possibility to observe the temperature rise inside and outside the meshing cycle

Cavitation erosion in liquid nitrogen

Thermodynamic effects in cavitation become significant only when the critical-point temperature is close to the operating temperature of the fluid, as in the case of cryogenic fluids. Therefore, the understanding and the prediction of the cavitation effects in such cases is crucial in many applications - for example the turbopumps for liquid hydrogen (LH2) and oxygen (LOX) in space launcher engines. The new generation of rocket engines will also feature the possibility of re-ignition while in orbit and prolonged period of operationhence cavitation erosion is becoming an issue at the design stage of the turbo-pumps. In the study, we show measurements of cavitation erosion in liquid nitrogen (LN2), where cavitation was generated by an ultrasonic transducer. The damage was evaluated on aluminium samples. Special care was given to accurate setting of the operation point - especially the operating pressure, which defines the size of cavitation. We show that it is less aggressive than cavitation in water and that its aggressiveness cannot be described by a single fluid property (for example the most commonly used Brennen\u27s thermodynamic parameter Σ), but by a combination of several (viscosity, density, vapor pressure, surface tension, thermodynamic parameter) - in the present paper we addressed this point by a simple bubble dynamics model with consideration of the thermodynamic effect to qualitatively predict the results of the measurements. Finally, we also compared performance of several other engineering materials

High-speed camera based optical measurement methods for in-mesh tooth deflection analysis of thermoplastic spur gears

The engineering characteristics of polymer gears, such as wear and fatigue resistance, are dictated by the underlying processes that take place during the gear meshing phase. These are in turn dependent on the running load, speed, temperature, lubrication regime and other parameters. So far, the structural response of a meshing gear could only be analysed using numerical approaches, typically involving the finite element method. In this study, an optical experimental analysis approach is proposed to evaluate the in-mesh gear tooth deflection based on high-speed camera recordings. For this purpose, two image processing methods were implemented, namely the well-known Digital image correlation method and a newly proposed Edge displacement detection method. Using an experimental setup with a camera resolution of px, an exposure time of 40 μs, a frame rate of fps and a 2000lm focused light source, a measurement accuracy of μm could be achieved. A good correlation between both methods and finite element analysis results was confirmed. Furthermore, the newly proposed Edge displacement detection method offers high potential for experimental analysis of long term wear and strain accumulation processes

Anomalies detected during hydrodynamic cavitation when using salicylic acid dosimetry to measure radical production

Cavitation used to be associated with negative outcomes in hydraulic turbomachinery but nowadays it is often used for water cleaning, microorganism\u27s destruction and degradation of organic compounds. This study investigated the amount of OH formed during hydrodynamic cavitation using salicylic acid dosimetry. The radical\u27s amount was evaluated by quantifying the concentration of 2,3-dihydroxybenzoic acid, catechol and 2,5-dihydroxybenzoic acid. Two concentrations of the dosimeter in tap water were investigated, 50 and 300 mg L-1 (pH approx. 2.5). After 90 min of cavitation using a Venturi constriction a sum of the three products was determined at 0.97 ug mL-1 and 1.81 ug mL-1, respectively. However, during the investigation the anomalies were detected in the cavitation development when higher concentration of salicylic acid was used - cavitation appeared more gentle, with less intense collapses, unrelated to the one in pure water. Detailed observations of cavitation and additional bubble dynamics simulations revealed that the decreased surface tension of the acidified salicylic acid solution is the most influential physical characteristic. Further experiments on nucleation and coalescence showed that high concentration of salicylic acid also leads to longer stability of the bubbles and prevents their coalescence due to short-range repulsive forces (steric hindrance), which results in less violent bubble collapse. We also discuss the importance of an appropriate amount of the dosimeter for correct evaluation of OH production in a given cavitation device (50 mg L-1 for the present one). This is essential for further cavitation exploitation studies to avoid false interpretation of the gathered results

Surface functionalization by nanosecond-laser texturing for controlling hydrodynamic cavitation dynamics

The interaction between liquid flow and solid boundary can result in cavitation formation when the local pressure drops below vaporization threshold. The cavitation dynamics does not depend only on basic geometry, but also on surface roughness, chemistry and wettability. From application point of view, controlling cavitation in fluid flows by surface functionalization is of great importance to avoid the unwanted effects of hydrodynamic cavitation (erosion, noise and vibrations). However, it could be also used for intensification of various physical and chemical processes. In this work, the surfaces of 10-mm stainless steel cylinders are laser textured in order to demonstrate how hydrodynamic cavitation behavior can be controlled by surface modification. The surface properties are modified by using a nanosecond (10-28 ns) fiber laser (wavelength of 1060 nm). In such a way, surfaces with different topographies and wettability were produced and tested in a cavitation tunnel at different cavitation numbers (1.0-2.6). Cavitation characteristics behind functionalized cylindrical surfaces were monitored simultaneously by high-speed visualization (20,000 fps) and high frequency pressure transducers. The results clearly show that cavitation characteristics differ significantly between different micro-structured surfaces. On some surfaces incipient cavitation is delayed and cavitation extent decreased in comparison with the reference - a highly polished cylinder. It is also shown that the increased surface wettability (i.e., hydrophilicity) delays the incipient cavitation

Application of (super)cavitation for the recycling of process waters in paper producing industry

In paper production industry, microbial contaminations of process waters are common and can cause damage to paper products and equipment as well as the occurrence of pathogens in the end products. Chlorine omission has led to the usage of costly reagents and products of lower mechanical quality. In this study, we have tested a rotation generator equipped with two sets of rotor and stator assemblies to generate developed cavitation (unsteady cloud shedding with pressure pulsations) or supercavitation (a steady cavity in chocked cavitation conditions) for the destruction of a persistent bacteria Bacillus subtilis. Our results showed that only supercavitation was effective and was further employed for the treatment of waters isolated from an enclosed water recycle system in a paper producing plant. The water quality was monitored and assessed according to the chemical (COD, redox potential and dissolved oxygen), physical (settleable solids, insolubles and colour intensity) and biological methods (yeasts, aerobic and anaerobic bacteria, bacterial spores and moulds). After one hour of treatment, a strong 4 logs reduction was achieved for the anaerobic sulphate reducing bacteria and for the yeastsa 3 logs reduction for the aerobic bacteriaand a 1.3 logs reduction for the heat resistant bacterial spores. A 22% reduction in COD and an increase in the redox potential (37%) were observed. Sediments were reduced by 50% and the insoluble particles by 67%. For bacterial destruction in real industrial process waters, the rotation generator of supercavitation spent 4 times less electrical energy in comparison to the previously published cavitation treatments inside the Venturi constriction design. In paper production industry, microbial contaminations of process waters are common and can cause damage to paper products and equipment as well as the occurrence of pathogens in the end products. Chlorine omission has led to the usage of costly reagents and products of lower mechanical quality. In this study, we have tested a rotation generator equipped with two sets of rotor and stator assemblies to generate developed cavitation (unsteady cloud shedding with pressure pulsations) or supercavitation (a steady cavity in chocked cavitation conditions) for the destruction of a persistent bacteria Bacillus subtilis. Our results showed that only supercavitation was effective and was further employed for the treatment of waters isolated from an enclosed water recycle system in a paper producing plant. The water quality was monitored and assessed according to the chemical (COD, redox potential and dissolved oxygen), physical (settleable solids, insolubles and colour intensity) and biological methods (yeasts, aerobic and anaerobic bacteria, bacterial spores and moulds). After one hour of treatment, a strong 4 logs reduction was achieved for the anaerobic sulphate reducing bacteria and for the yeastsa 3 logs reduction for the aerobic bacteriaand a 1.3 logs reduction for the heat resistant bacterial spores. A 22% reduction in COD and an increase in the redox potential (37%) were observed. Sediments were reduced by 50% and the insoluble particles by 67%. For bacterial destruction in real industrial process waters, the rotation generator of supercavitation spent 4 times less electrical energy in comparison to the previously published cavitation treatments inside the Venturi constriction design

Visualization and measurements of shock waves in cavitating flow

Upon cavitation cloud collapse an omnidirectional shock wave is emitted. It then travels through the flow field, causing a cascade of events resulting in erosion, noise, vibration and the cavitation shedding process. Despite that the accumulated data points evidently to the presence of the shock waves, the direct measurements hardly exist - and even then, they are very expensive and time consuming to perform. In the present paper, the possibility of detecting shock waves inside cavitating flow is shown. The methodology bases on using two conventional high speed cameras. With the first one cavitating flow from a distance is observed, determining the position of the wave, while the second camera with a microscopic lens enables a close-up view to determine the number and size change of air bubbles as a shock wave passed them. By calibration and reference measurements the amplitude of the shock waves was determined. This relatively simple approach enabled the first observation of shockwaves which occur at the cavitation cloud collapse (downstream of the attached cavity). Several examples of shock wave dynamics are shown and how they influence the general cavitation cloud behaviour. Shock wave front velocities and local pressure waves caused by cloud collapse were estimated from visualization, reaching values to more than 700 m/s and over 5 MPa respectively