During this experiment, we identified the behaviour of water during the transition between liquid and vapour phases, at temperature around the vapor point over a range of pressures. We need to observe fluid appearance using a sight glass set into the pressure vessel to study the change in vapor point with increasing pressures.
We did saw the bubble swirling along with water at the sight glass. There, we did studied behaviour of water, at temperature around the vapour point, over a range of pressures. At the first 5 minutes, the temperature is 39.5oC and pressure with 100kN/m2, water was heated but no boiling occurred at very low heat flux between the heater and the fluid. We identified the phases change only occur at the surface of water. So, the water undergo evaporation and only water at surface vaporised. When heat flux increase, the pressure reading increase with temperature. At this state, water droplet started to cover the sight glass and slight swirling can be seen too. Two-phase flow can occur in various forms, such as flows transitioning from pure liquid to vapor as a result of external heating, separated flows, and dispersed two-phase flows where one phase is present in the form of particles, droplets, or bubbles in a continuous carrier phase.3 At the heater surface, increased of heat flux produce many small bubbles of vapour forming as the layer of fluid surrounding it. These happened when the hot boundary layer rose out between the heater and the main volume of the water, until they reach cooler fluid, where they condensed.
When the temperature reaches 100oC, the movement of bubble inside agitated fluid, increased the mixing. The boiling point of water is 100oC. We termed it as nucleate boiling where the bubbles movement was too vigorously. In constant volume, the heating was continued going up. The internal pressure of the system increases as the saturation temperature of water also increase. The vapor became superheated because its temperature was higher than boiling point at atmospheric pressure. The temperature of water is increases as the saturation pressure increases that made the liquid in the system become superheated which it remains liquid at a temperature above the boiling point at atmospheric pressure.
The saturation pressure of water was measured using a pressurized vessel. We found out that pressure is proportional with temperature when heated. The fluids behaviours of gas-liquid two-phase fluid was assumed before that the transition of liquid to vapour was strongly effects on heat and flow characteristics, air-water isochoric two-phase fluids where volume was constant throughout the experiment. This is because the result show at current point where pressure change suddenly oppose the rising of the temperature because the water was totally saturated with pressure. This means saturated water pressure at the line between liquid and vapour may cause change phase in just small portion in any further addition of energy. Likewise, along the saturated vapor line, any removal of energy will cause some of the vapor to condense back into a liquid, producing a mixture. When a substance reaches the saturated liquid line it is commonly said to be at its boiling point.
Since we used water which is pure substance as our liquid in this study, it can exist in many phases which is compressed liquid. Under these conditions, water exists in the liquid phase, and it is called a compressed liquid, or subcooled liquid, meaning that it is not about to vaporize. Heat is now transferred to the water until its temperature rises to, say, 40oC. As the temperature rises, the liquid water expands slightly, and so its specific volume increases. As from analysis, at temperature 39.5oC with pressure 1.0 bar, we saw no bubble can be seen through sight glass and this fluid appearance was in liquid state. Besides, water also can be saturated liquid where a small change in temperature or pressure can cause the substance to vaporise.
From the result of experiment, temperature at 101.5oC with 1.9 bar, the characteristic of water can be clearly seen where more bubbles were moved vigorously. As pressure increases, the saturation temperature of water also increased and the liquid in the system become superheated. The chemical properties of water are also greatly changed at high temperatures and pressures due to the changes in dissociation, solubility, diffusivity, and reactivity due to decreasing hydrogen-bonding.4 Relate this statement, if the pressure is now reduced without a corresponding reduction in temperature, for example by bleeding off steam from the system, then the saturation temperature is reduced. The superheated liquid vaporises as the vapour point falls, producing violent frothing. During a boiling process, the only change we will observe is a large increase in the volume and a steady decline in the liquid level as a result of more liquid turning to vapor.
At 128.5°C and 330kN/m2, the bubbles were in turbulent motion even after there is less pressure when calorimeter valve was partially opened. Lastly, the bubbles were in less turbulent motion at 123.8°C. At this state, the pressure decreased would made temperature decline. The water was cooled down while the volume was constant because the change of the pressure made the boiler condensed with water vapor. It made temperature of boiler decline.
Suggestion and recommendation
From experiment, before we start the experiment, it is important to make sure that pressure valve 1 and pressure valve 2 are closed when turned on the heater. This is because to control pressure by make sure the reading increase as the heat flux increase. Then, we should fill up the water half of the boiler to allow the observation through the sight glass with bubbles and water vapor. Besides, we also need to alert for taking reading in every 30 seconds as the temperature and pressure increase. So, we can fix our data which experiment result with actual result. Moreover, it is necessary to install pump between input of boiler within the entrance of water. This is because to avoid water splitting when filling up the water into a boiler with a hose.
In addition, the boiler should be installed with overpressure detector so that it can be a backup for any disfunction of pressure valve. The overpressure detector installation would show a red light for warning if pressure in boiler is saturated and overstress.
1. C. Tribe and H. Müller-Steinhagen, Heat Transfer Engineering, 22,1, pp.12–21, 2001.CrossRefGoogle Scholar
Cite: Jiang L., Asano H., Takenata N., Sugimoto K. (2007) Heat Transfer Characteristics of Boiling Two-Phase Flow in a Plate Heat Exchanger. In: Cen K., Chi Y., Wang F. (eds) Challenges of Power Engineering and Environment. Springer, Berlin, Heidelberg
2. Moran, Michael J., and Howard N. Shapiro. Fundamentals of Engineering Thermodynamics. 7th ed. Hoboken, N.J.: Chichester: Wiley; John Wiley, 2011. Print.
3. Ghiaasiaan, S. M.Two-Phase Flow, Boiling, and Condensation: In Conventional and Miniature Systems, Cambridge University Press, 2008. pg 362.
4. A., k., & E., D. (2007). Hot compressed water as reaction medium and reactant. Properties and synthesis reactions, 362-380.
5. Contributors, W. (2017, October 22). Two-phase flow. Retrieved from https://en.wikipedia.org/wiki/Two-phase_flow