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ISSN : 1598-5504(Print)
ISSN : 2383-8272(Online)
Journal of Agriculture & Life Science Vol.50 No.4 pp.203-211
DOI : https://doi.org/10.14397/jals.2016.50.4.203

Heating Performance of a Heating Unit Using Hydrodynamic Cavitation

Dae Bin Song1*, Byeong Gyu Kang2
1Department of Bio-Industrial Machinery Engineering, Gyeongsang National University(Institute of Agric. & Life Sci.), Jinju, 52828, Korea
2Hankuk Magneto Co., Ltd., Jinju, 52781, Korea
Corresponding author: Dae Bin Song +82-55-772-1895+82-55-772-1899dbsong@gnu.ac.kr
October 27, 2015 April 21, 2016 July 20, 2016

Abstract

This study examined the factors affecting the bubble generation of a motor driven bubble generator to develop a heating unit using hydrodynamic cavitation. This study also investigated the heat production and thermal efficiency by changing operating conditions. Bubble generation using the 25 ℓ-capacity motor is driven bubble generator was confirmed visually in various experimental conditions: three levels of motor powers(1, 3, 5 HP), two levels of revolutions(1800, 3200 rpm), and two levels of internal pressures of the bubble generator(the atmospheric pressure, pressurized air). After constructing the heating unit, heat production, and thermal efficiency were measured in the following experimental conditions: two levels of motor powers(3, 5 HP) and three levels of water quantities(102, 152, 230 kg). And then specifically temperature increasing rate and specific consumed energy required for the heating unit design were calculated. Bubbles were generated stably at 1,800 rpm and pressure from 0~0.8 bar. When heating water around 30°C, specific temperature increasing rate was maximized at 0.247°C/min and 0.002422°C/min-kg. Thermal efficiencies were 121% with only motor driving power as input energy and 98% with both motors driving power and water circulating pump driving power as input energy. This showed that the heating unit using hydrodynamic cavitation had higher thermal efficiency than the existing combustion boiler. Maximum specific consumed energy was 0.0270 KJ/min-kg-°C. This study confirmed that water can be heated with the heat caused by the explosion of the bubbles generated by hydrodynamic cavitation. And the results of this study could be utilized for commercial use because it showed much higher thermal efficiency than the existing combustion boiler.


초록


    Introduction

    Vapor cavities in a liquid are generated when the ambient pressure drops below the vapor pressure due to the increased flow velocity. The vapor cavities join together and grow, and they implode when they move to the high pressure area. High temperature and high pressure are generated instantaneously when the voids implode, and this is called cavitation(Namkung, 2007). At this point recovering latent heat of vaporization and releasing thermal energy by adiabatic compression are occurred, and the released energy heats the ambient liquid. Details of thermal energy releasing by the voids implosion are as follows:

    • Recovering the latent heat of vaporization: 540 kcal/kg(if the voids condense not implode)

    • heat generation by high temperature(5,000 K) generated during the implosion

    • serial radical reaction by high pressure(1,000 atm) generated during the implosion and heat generation by hydrogen gas explosion generated during the radical reaction(Ince et al., 2001)

    H2O → ·OH + ·H(water molecules split), 2H· → H2

    Heating by hydrodynamic cavitation does not produce pollutants and carbon dioxide because it does not burn fossil fuels. However, cavitation causes damages and corrosions on the surface of a hydraulic machine, therefore it is considered as an undesirable phenomenon(Li, 2000).

    Study on vortex thermal generators conducted in Russia during the 1930s is the representative research on heating liquid using hydrodynamic cavitation (Zaporozhets et al., 2004). However, they provided patents and commercial data for the vortex thermal generators but did not include data on product performance such as thermal efficiency(Kwon et al., 2010).

    Little.(2006) reported that the thermal efficiency of a vortex thermal generator was 80%, but detailed experiment method and data were not provided. Kwon.(2011) conducted an experiment of the vortex thermal generator using 15 and 55 kW electric motors and reported that thermal efficiencies having input energy of motor power consumption except the power consumption of the pressurized pump for water circulation were 90.42%(15 kW) and 93.89%(55 kW).

    The cavitation bubbles are generated by ultrasonic vibrations or reducing the vapor pressure by using flow rate. Ultrasonic vibrations create bubbles with cold liquids but does not generate with saturated liquids. On the other hand, reducing the vapor pressure creates bubbles easily by mechanical rotation of an object; therefore, significant economic effect is expected to use electric motors for the mechanical rotation compared to the existing method using the combustion fuel for heating water.

    This study examined the factors affecting the bubble generation of a motor is driven bubble generator to develop a heating unit using hydrodynamic cavitation. This study also investigated the heat production and thermal efficiency by changing operating conditions.

    Materials and Methods

    1.Cavitation generator

    A bubble generator was designed as Figure 1, which was composed of an electric motor for the rotor drive, coupling, a drive shaft, a rotor for bubbling, a pail, and a base for the device. The pail used acrylic tube of a capacity of about 25 ℓ, and the base was configured to be used with three types of motors(1, 3, 5 HP). The experimental unit shown in Figure 2 consisted of a water tank for water supply and discharge, a valve for controlling inside pressure of the pail, a pressure gage, a water pump for circulation(PU-602M, 170 ℓ/min., Wilo pump CO., LTD, Korea), a thermo couple(Pt-100Ω), and transparent rubber hose.

    2.Heating unit

    A water heating system using hydrodynamic cavitation was designed as Figure 3. The system was composed of the bubble generator, the water pump for circulation, and the water tank for water supply and discharge; each device was connected with transparent rubber hose. The bubble generator had a 25 ℓ pail and frame to be fitted three types of motors(1, 3, 5 HP) depending upon the conditions. The water pump(PU-602M, 170 ℓ/min., wilo pump CO., LTD, Korea) was used for water circulation, and the 200 ℓ rectangular water tank was made with stainless steel. The thermocouple(Pt-100Ω) was attached on the tank to measure the temperature of supplied and discharged water. The valve for discharging water completely after the experiment and the pressure gage(0~10 MPa) were also attached. Connecting hose and plumbing hose were insulated for preventing heat loss due to heat transfer to the outside. Figure 4 depicts the manufactured water heating system.

    3.Cavitation generator experiment

    After filling the water tank, water was supplied to the pail using the water pump. The pail was completely filled with water, and then the motor was started. Bubble generation was confirmed visually in the following experimental conditions: three levels of motor capacities(1, 3, 5 HP), two levels of revolutions(1800, 3200 rpm), and two levels of internal pressures of bubble generator(the atmospheric pressure, pressurized air). The pressure was adjusted using a flow control valve attached to the outlet at atmospheric pressure(0 bar) and pressurized air(0.05~0.1 bar).

    4.Heating unit experiment

    Heat production and thermal efficiency were measured with the experimental conditions of two levels of motor capacities(3, 5 HP) and three levels of water quantities(102.6816, 152.6816, 209.0317 kg). Water temperature and lab temperature were measured with a thermocouple(Pt-100Ω), and two different power consumptions were measured with a power meter(PW3360, HIOKI, Japan). One was rotor driving power consumption for bubble generation, and the other was pump driving power consumption for water circulation. Water temperature, lab temperature, and power consumption were measured for each condition after operating the device at 10-min intervals, and input energy, generated heat, and thermal efficiency were calculated as follows:

    Input energy(kcal)= input power(kwh) × 860(kcal/kwh)

    Generated heat(kcal)= temperature increase(°C) × specific heat(kcal/°C – kg) x water quantity(kg)

    Thermal efficiency I(%)= generated heat(kcal) × 100 / (motor + pump) power consumption(kcal)

    Thermal efficiency II(%)= generated heat(kcal) × 100 / motor power consumption(kcal)

    Results and Discussion

    1.Cavitation generator experiment

    Table 1 shows the results of the bubble generation based on the motor power and the revolutions. Bubble generation at 1800 rpm was stable regardless of motor power. However, at 3200 rpm the device failed to work simultaneously with motor driving due to overload, but only for a short time, it created bubbles in large quantities. The results based on the internal pressure of the pail represented that the bubbles generated at the atmospheric pressure decreased with increasing pressure and completed at 0.8 bar.

    2.Heating unit experiment

    Figure 5 describes the changes of water temperature, lab temperature, and thermal efficiency of the heating system with 102 kg water and 3 HP motor power over time. Water temperature for 300 minutes increased from 6.3°C to 69.0°C. However, it decreased slowly around 30°C due to the loss caused by heat transfer to the outside air. Lab temperature was maintained around 19 ~ 23°C. Thermal efficiency I which had motor and pump driving powers for input energy showed a tendency of decreasing from 115% to 80%, and it continuously decreased from 100% at 30°C. Thermal efficiency II which had motor driving power for input energy decreased from 138% to 95%, and the differences were 23%(maximum) and 15%(minimum). In thermal efficiency of the boiler using the fuel for combustion, only heating value of the combustion fuel not including pump driving power was considered as input energy. This is because this study calculated thermal efficiency in two different ways. Thermal efficiency I and II at water temperature of 30°C before heat loss generating were 98.3% and 118.3%, respectively, which showed higher values than the one of combustion boiler.

    Figure 6 gives the changes in water temperature, lab temperature, and thermal efficiency of the heating system with 152 kg water and 3 HP motor power over time. Considering the heat loss of water temperature, the experiments were conducted for 180 minutes from 5°C to 36.9°C. Lab temperature was maintained around 13 ~ 23°C. Slow decreasing was observed from 109% to 99% at thermal efficiency I and from 129% to 117% at thermal efficiency II, and the differences were 20%(maximum) and 18% (minimum). Final thermal efficiency I and II were 99% and 117%, respectively, which showed higher value than the one of combustion boiler.

    Figure 7 provides the changes of water temperature, lab temperature, and thermal efficiency of the heating system with 230 kg water and 3 HP motor power over time. Considering the heat loss of water temperature, the experiments were conducted for 220 minutes from 11.8°C to 35.6°C. Lab temperature was maintained around 11.8 ~ 21.5°C. Thermal efficiency increased for early 50 minutes and then decreased. This is because power consumption increased due to the relaxation of rotor bearing assembly after 50 minutes. Slow increasing was observed from 99% to 100% at thermal efficiency I and from 119% to 121% at thermal efficiency II, and the difference was 20% throughout the experiment. Final thermal efficiency I and II were 100% and 121%, respectively, which showed higher value than the one of combustion boiler.

    Figure 8 shows the changes of water temperature, lab temperature, and thermal efficiency of the heating system with 152 kg water and 5 HP motor power over time. Considering the heat loss of water temperature, the experiments were conducted for 180 minutes from 5.6°C to 36.2°C. Lab temperature was maintained around 17 ~ 21°C. Slow decreasing was observed at thermal efficiency I from 127% to 98% and at thermal efficiency II from 157% to 118%, and the differences were 30%(maximum) and 20%(minimum). Final thermal efficiency I and II were 98% and 118%, respectively, which showed higher value than the one of combustion boiler, and they were also similar to the one with 152 kg and 3 HP.

    The results represented that thermal efficiencies of the heating device using hydrodynamic cavitation were observed evenly regardless of motor power and water quantity. Based on the water temperature 30°C, maximum thermal efficiency I was 100% and minimum was 98%. In addition, maximum thermal efficiency II was 121% and minimum was 117%. Both showed higher value than the one of combustion boiler, therefore, it is expected to be applicable for commercial use.

    Figure 9 gives the power consumption of each experimental condition at an interval of 10 minutes. Power consumption to generate bubbles with a capacity of 25 ℓ for 10 minutes was in between 0.30 kWh ~ 0.33 kWh regardless of water quantity and motor power, and it will increase with increasing capacity of the device. 3 and 5 HP motors showed the same amount of power consumption, which represented that the power was used only for bubble generation not heating water. This can be said that water was heated by hydrodynamic cavitation.

    Figure 10 represents the specific temperature increasing rate converting temperature increasing in terms of time and water quantity at each experimental condition. To eliminate the effects of external heat loss, heat production was calculated based on time of heating water to around 30°C. Heat production per min. decreased proportionately with increasing quantity, and it was same in the same quantity regardless of motor power. The specific temperature increasing rate of 102 kg was 0.247 °C /min, 0.002422 °C/min-kg which was lower than the one from combustion boiler, and this was major problem of the heating unit using hydrodynamic cavitation.

    Figure 11 shows the specifically consumed energy converting consumed energy in terms of time, quantity, and temperature increasing at each experimental condition. The specifically consumed energy was 0.0270 KJ/min-kg-°C with 102 kg and 3 HP and 0.0233 KJ/min-kg-°C with 230 Kg and 3 HP, and the difference was similar to 15.8%. Same values were observed with same quantity. Specific consumed energy presented from this study can be used as a useful design data to calculate the input energy for quantity, heat production, and time in designing the heating unit.

    Figure

    JALS-50-4-203_F1.gif

    Schematic diagram of the experimental cavitation generator unit.

    JALS-50-4-203_F2.gif

    Photograph of the experimental cavitation generator unit.

    JALS-50-4-203_F3.gif

    Schematic diagram of the experimental heating unit.

    JALS-50-4-203_F4.gif

    Photograph of the experimental heating unit.

    JALS-50-4-203_F5.gif

    Temperature & thermal efficiency comparison (3 hp, 102 kg).

    JALS-50-4-203_F6.gif

    Temperature & thermal efficiency comparison (3 hp, 152 kg).

    JALS-50-4-203_F7.gif

    Temperature & thermal efficiency comparison (3 hp, 230 kg).

    JALS-50-4-203_F8.gif

    Temperature & thermal efficiency comparison (5 hp, 152 kg).

    JALS-50-4-203_F9.gif

    Heating ability comparison.

    JALS-50-4-203_F10.gif

    Heating ability comparison.

    JALS-50-4-203_F11.gif

    Specific energy consumption comparison.

    Table

    Bubble generation

    Reference

    1. Ince NH , Tezcanli G , Belen RK , Apikyan IG (2001) Ultrasound as a catalyzer of aqueous reaction systems: the state of the art and environmental applications , Applied Catalysis B. Environmental, Vol.29 (3) ; pp.167-176
    2. Kwon CH , Lee JH , Kwon BP , Yoon JY (2010) The performance of the vortex thermal generator according to the flow conditions , Proceedings of the KFMA Annual Meeting Mokpo Korea, ; pp.158-159
    3. Kwon WC (2011) Design and Performance Analysis for a Heat Generator Using Hydrodynamic Cavitation , PhD diss. Seoul Korea Hanyang University Department of Mechanical Engineering, In Korean with English abstract
    4. Li SC (2000) Cavitation of Hydraulic Machine, Imperial College Press,
    5. Little SR (2006) Null tests of breakthrough energy claims 2nd AIAA/ASME/SAE/ASEE,
    6. Namkung KC (2007) The principle of hydrodynamic cavitation and its applications to water treatment , The Magazine of the Korean Society of Civil Engineers, Vol.55 (10) ; pp.47-52
    7. Zaporozhets EP , Kohlpanav LP , Zibert GK , Artemov AV (2004) Vortex and cavitation flows in hydraulic systems , Theoretical Foundations of Chemical Engineering, Vol.38 (3) ; pp.225-234
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