Thermal conductivity measurement of two microencapsulated phase change slurries

Transcription

1 Thermal conductivity measurement of two microencapsulated phase change slurries Xiaoli Ma (corresponding author), Siddig Omer, Wei Zhang and S. B. Riffat Institute of Sustainable Energy Technology, School of the Built Environment, University of Nottingham, Nottingham, NG7 2RD, UK Abstract This paper presents results of a thermal conductivity test carried out on microencapsulated phase change materials, to be used as a cooling storage and transport medium in a solar-driven air conditioning system. The microencapsulated slurries can be used not only as an energy storage medium but also as an energy transport medium, and this helps the system to be designed as compact as possible. The test has been carried out on two recently developed microencapsulated phase change materials. A convenient measurement system using transient line heat source technique is introduced, and the results of these measurements are presented in this paper. Keywords microencapsulated phase change slurry; thermal conductivity; thermal properties meter Nomenclature k thermal conductivity (Wm 1 K 1 ) D thermal diffusivity (m 2 /s) t time (s) q rate of heat dissipation (W/m) t 1 heating time Ei exponential integral 1. Introduction Utilizing microencapsulated phase change slurries (MEPCM slurries) in energy storage systems, heat exchangers and thermal control systems is a new technique. The MEPCM slurries are microencapsulated and suspended in a conventional singlephase heat transfer fluid to form phase change slurries. Such slurries have large apparent specific heats during the phase change period, which enhances the heat transfer rate between the fluid and the tube wall [1]. Therefore, they could have many potential important applications in the areas of heating, ventilation and airconditioning, refrigeration and heat exchange, etc. When designing the cold storage system using the MEPCM slurry, thermal conductivity of the MEPCM slurry is essential. In order to measure the thermal conductivity of a liquid, one of the most reliable and well-known methods is a transient line heat source technique. Using this technique, a thin line heat-source (hot wire) is stretched vertically within the sample, and the thermal conductivity of the sample is determined from the transient response of the line heat source, with the constant heat generation per unit length [2].

2 246 X. Ma et al. The key issue of measuring the thermal properties of fluids using the transient line heat source technique is the influence of the convective heat exchange including free convection and forced convection. Forced convection occurs when the fluid is agitated or mixed by mechanical forces, which is easy to avoid. Free convection may occur when a body of higher or lower temperature is inserted into a fluid; therefore, the sensor (heat source) itself can cause the free convection. To solve this problem, low viscosity fluids must be stabilized, or complicated mathematical corrections are needed when using a common transient line heat source sensor. In this work, we used a KD2 Pro KS-1 needle sensor having a very small heat pulse that can measure thermal conductivity of most fluids without causing free convection. Therefore, stabilizing the thin fluids and subsequently, mathematical corrections are not needed. A measuring rig can be set up to obtain various sample temperatures whilst avoiding both the forced and free convection during the measurement process. The thermal conductivity of the water measurement at 20ºC was W/m.K, and compares very well with data from reference [3] at the same temperature i.e., (0.613 W/m.K). The thermal conductivities of two MEPCM slurries (DPNT and DPNT ), which can be used for cooling systems have been measured under various temperatures and concentration ratios, and the results are presented in this paper. 2. Measuring equipment and methodology 2.1 Thermal property meter KD2 Pro Thermal properties meter Thermal conductivity measurement of liquids is a very difficult process and requires very special and expensive equipment. These are not accessible to many people. A KD2 Pro Thermal properties meter (Decagon Devices Inc) has been used for the measurement of thermal conductivity of MEPCM slurries considered in this work. The KD2 Pro can measure the thermal conductivity of liquid using the transient line heat source method. With this technique, a 30 second heat pulse is applied to a needle, and the temperature response with time is monitored either at the heated needle or at an adjacent needle. The nature of the temperature response is a result of the thermal properties of the material. The specification of the KD2 Pro used for testing the thermal conductivity of the liquid is as follows: Sensor used: KS-1 6 cm sensor Measurement range: 0.02 to 2.00 W/m.K Accuracy: ±5% from W/m.k; ±0.01 W/m.k from W/m.K Operating Environment: 50ºC to 150ºC KD2 Pro Theory The temperature surrounding an infinite line heat source with constant heat output and zero mass in an infinite medium was modelled. When a quantity of heat, Q (Jm 1 ) is instantaneously applied to the line heat source, the temperature rise at distance, r (m) from the source is given by [4]:

3 Thermal conductivity measurement of two microencapsulated phase change slurries Q r ΔT = 4πkt exp 4Dt (1) If a constant amount of heat is applied to a zero mass heater over a period of time, rather than as an instantaneous pulse, the temperature response is given by [4]: 2 ΔT qk Ei r = Dt 0 < t t 1 (2) 4π 4 The temperature rise after the heat is turned off is given by [4]: 2 2 q r r ΔT = Ei Ei k Dt + D( t t ) t > t 1 (3) 4π 4 4 Material thermal properties are determined by fitting the time series temperature data during heating to Eq. (2), and during cooling to Eq. (3). Thermal conductivity can be obtained from the temperature of the heated needle, with r taken as the radius of the needle. 2.2 Test rig set up and measurement method When measuring thermal properties only the heat transfer resulting from conduction is needed. In non-viscous fluids, heat transfer by convection can be much greater than heat transfer by conduction. Hence, accurate measurement of the thermal properties in fluids requires that convective heat exchange be negligible. The KD2 Pro KS-1 needle sensors have a very small heat pulse that can measure the thermal conductivity of most fluids without causing free convection. Due to this property, the KD2 Pro KS-1 sensor can accurately measure the thermal conductivity of fluids as thin as water, without being stabilized, if it is oriented vertically in the water. The aim is to minimise the characteristic dimension of the probe inserted into the fluid in order to reduce free convection. This is because the heat conductance by free convection is inversely related to the characteristic dimension of the probe inserted into the fluid. For a probe with its axis parallel to the fluid flow, the characteristic dimension is its length. For a probe with its axis perpendicular to the fluid flow, the characteristic dimension is its diameter. To measure the thermal conductivities of the two MEPCM slurries at various temperatures (below the ambient temperature), a measuring rig has been set up (see Fig. 1). The rig can not only cool the fluid to the desired temperatures, but also eliminate convective heat transfer during the measurement period. As shown in Fig. 1, the chilled water from a temperature controlled chiller flows through the helix coil, which will cool the water in the tank. The tank is insulated using 20 mm armaflex insulation sheet. A 226 ml (6 cm diameter and 8 cm height) aluminium cup that contains the MEPCM slurry is attached to the internal surface of the cover. The MEPCM slurry in the aluminium cup is cooled by the cooled water 1

4 248 X. Ma et al. Figure 1. Measurement rig set up (a) Schematic drawing of the measurement rig; (b) Photos of the measurement rig. in the tank. The KS-1 sensor that connected to the KD2 Pro thermal properties meter is inserted into the MEPCM slurry vertically through a hole in the cover to minimize errors from free convection. A thermal meter is used to monitor the temperature inside the tank and the temperature inside the aluminium cup. The temperature difference between the sample and the water was controlled within 0.01ºC before the

5 Thermal conductivity measurement of two microencapsulated phase change slurries 249 data was taken. Five minutes before taking the measurement, the sample was stirred using a flexible fibre through a small hole in the cover, to make the sample s temperature in the aluminium cup uniform. As the cover of the tank is well fixed, there is no shaking or vibration during the measurement period. Before taking the measurement the chilled water was switched off. There is no heating or cooling source inside the tank. The sample temperature can be kept constant by the well-insulated tank during the measurement period (90 seconds). The central part on the cover of the tank was insulated as well during the measurement process. The sensor was always kept in the sample to allow the sample and sensor to come to a uniform temperature before the measurements were taken. The data was recorded at least every 30 minutes to allow time between the readings for the temperatures to reequilibrate. A correlation coefficient, which is a measure of how well the model of the measurement fits the data, is shown together with the test results (including temperature and thermal conductivity) on the meter, in order to check the measured result. 3. Specifications of MEPCM Material (DPNT and DPNT ) Two MEPCM slurries (DPNT and DPNT ) have recently been developed by Ciba UK plc. The two MEPCM slurries are small spherical microcapsules of an average mean particle diameter of 2.0 mm, comprising of an acrylic polymer shell ( 100 nm thick) surrounding a paraffin wax core, dispersed in water. The two MEPCM slurries have different melting points: The DPNT has a melting point of around 6ºC and the DPNT has a melting point of around 15ºC. The two MEPCM slurries are proposed for use in a cooling storage system. 4. Results and discussion The measurement results for DPNT and DPNT at concentration ratio of 45%, 30%, 25% and 20% have been recorded. The choice of these four concentrations is based on the fact that the concentration ratio determines the transport and the storage capability of the MEPCM. The material was initially produced at a 45% concentration, but this is difficult to be pumped through the cooling system. Decreasing the concentration improves the pumping capabilities, but at the same time affects the storage capacity. The minimum concentration that favours the MEPCM from pure water is in the vicinity of 25%. The results obtained from the measurement period are presented in Fig. 2, Fig. 3, Fig. 4 and Fig. 5. The variation of the thermal conductivity with the temperature for DPNT and DPNT are shown in Fig. 2 and Fig. 3 respectively. It is found that during the phase change periods for both slurries, the thermal conductivities get much higher. This is because when the thermal conductivity of the MEPCM slurry is measured using a transient line heat-source technique, the measured value is affected by the latent heat of fusion. Therefore, the measured thermal conductivity should be apparent thermal conductivity. The apparent thermal conductivity for the

6 250 X. Ma et al. Figure 2. Thermal conductivity of DPNT versus temperature. Figure 3. Thermal conductivity of DPNT versus temperature.

7 Thermal conductivity measurement of two microencapsulated phase change slurries 251 Figure 4. Thermal conductivity of DPNT versus concentration. Figure 5. Thermal conductivity of DPNT versus concentration.

8 252 X. Ma et al. concentrations of 30%, 25% and 20% are higher than the thermal conductivity of water in a certain temperature range, as shown in Fig. 3. It is found that when the temperature is lower than the phase change temperature (the phase change material in the capsule is solid), the thermal conductivity increases with the temperature (the DPNT increases more quickly than the DPNT ). When the temperature is higher than the phase change period (the phase change material in the capsule is liquid), the thermal conductivities remains almost constant. The variations of thermal conductivity with concentration for certain temperatures for DPNT and DPNT are shown in Fig. 3 and Fig. 4 respectively. The temperatures include a temperature beyond the phase change period (3ºC for DPNT and 11ºC for DPNT ), a temperature within the phase change period (4.8ºC for DPNT and 14.5ºC for DPNT ) and a temperature over the phase change period (8ºC for DPNT and 18.5ºC for DPNT ). It is shown that for each temperature (beyond phase change temperature, within phase change period and over phase change temperature) the thermal conductivity decreases linearly with the increase of the concentration in the tested concentration range (20% to 45%). This is because when the temperature is out of the phase change range, the increase of the contents of water in the slurry increases the thermal conductivity. This is due to the fact that water has a higher thermal conductivity than the phase change material; when the temperature is within the phase change range, the increase of the water contents in the tested range (concentration reduces from 45% to 20%) increases the heat transfer of the slurry and the phase change speed increases. Therefore, the apparent thermal conductivity increases although the contents of the phase change material decrease. Of course, increase of the water contents itself during the phase change period also increases the thermal conductivity. 5. Conclusions The thermal conductivities for various temperatures and concentrations of the MEPCM slurries have been obtained using the KD2 Pro thermal properties meter. Using this equipment under well controlled measurement conditions ensures that accurate values of thermal conductivity can be obtained without the need for complex mathematical corrections and fluid stabilization. The results of DPNT and DPNT measurement show that thermal conductivities are affected by the latent heat of fusion when measured using the transient line heat source technique. Therefore, the apparent thermal conductivities have been obtained during the phase change period. The apparent thermal conductivities are higher than that of pure water when the slurry is diluted to below 30%, 25% and 20%. It was found that when the temperature is below the phase change period (the encapsulated PCM is solid) the thermal conductivity increases slightly with the temperature, whilst when the temperature is over the phase change period (the encapsulated PCM is liquid) the thermal conductivity remains constant. It was also found that for a fixed temperature, whether it is within the phase change period

9 Thermal conductivity measurement of two microencapsulated phase change slurries 253 or outside of it, the thermal conductivity decreases linearly with the MEPCM slurry concentration within the tested concentration range (20% to 45%). Acknowledgements This work is supported by The European Commission. Thank you to CIBA Specialty Chemicals plc for providing the MEPCM slurries. Reference [1] Xianxu Hu and Yingping Zhang, Novel Insight and Numerical Analysis of Convective Heat Transfer Enhancement with Microencapsulated Phase Change Material Slurries: Laminar Flow in a Circular Tube with Constant Heat Flux, International Journal of Heat and Mass Transfer, 45 (2002), [2] Koji Matsumoto and Takahiro Suzuki, Measurement of Thermal Conductivity of Ice Slurry Made From Solution by Transient Line Heat-Source Technique (Analytical Discussion on Influence of Latent Heat of Fusion), International Journal of Refrigeration, 30 (2007), [3] Zhuang Jun, Xutong Ming and Shi Shouchun, Heat Pipe and Heat Pipe Heat Exchanger, Shanghai Jiaotong University Publications, Inc., China, (1984). [4] KD2 Pro Thermal Properties Analyzer, Operator s Manual, Version 4, (2006) Decagon Devices, Inc, 2365 NE Hopkins Ct. Pullman, WA USA.