Phase Change Processes
The objective of the present experimental study is to investigate the thermal performance of a shell and tube type LHSU filled with paraffin wax and stearic acid as PCM. A critical evaluation of the PCM temperature profile with time is carried out. The analysis is carried out for different mass flow rates and fluid inlet temperatures of HTF during melting and solidification process.
In order to enhance thermal performance of LHSU, longitudinal fins are attached on the inner tube of HTF at PCM side. The heat transfer augmentation due to the fins is established in the experimental study in terms of melting and solidification time. An energy based thermal performance is also established for both with and without fin LHSU.
A test rig of shell and tube type LHSU is designed, fabricated and installed. Looking to a long term objective of development of LHSU for solar water heater application, appropriate geometric parameters, operating parameters and PCM are selected for the experimentation. An angle adjuster is provided to understand the influence orientation. Detailed experimental investigation has been carried out for seven angles (θ = 90º, 75º, 60º, 45º, 30º, 15º, 0º). In order to obtain comprehensive depiction of temperature variation in the PCM, total 45 K-type thermocouples are positioned in the annular space between the shell and tube where the PCM is placed. The arrangement of these thermocouples are such that measurement of PCM temperature is possible in all three directions (axial, radial and angular). Experimental analysis suggests that the charging process is dictated by natural convection. Natural convection varies as orientation changes. Thus, orientation influences the charging process significantly. LHSU oriented at θ = 45° takes minimum time for overall charging and storage of energy owing to natural convection driven melt flow. However, it is deduced that the upper half of the horizontal (θ = 0°) LHSU melts very rapidly. PCM average temperature, liquid fraction and energy storage rate are found highest for horizontal LHSU till the melt interface reaches the bottom of the HTF pipe. However, discharging process isconduction dominant hence orientation has no significant effect on discharging process of the PCM. Influence of HTF temperature on the charging and discharging rate is found to be significant compared to HTF mass flow rate
The heat transfer augmentation methods analysed include imparting eccentricity to Heat Transfer Fluid (HTF) pipe, imparting rotation to the LHSU and using multi HTF tube. When the inner HTF tube is eccentric by 10 mm below the axis, natural convection dominated region increases which leads to 29.35% reduction in the melting time. Rotating LHSU offers superior melting rate with about 47.75% reduction in the melting time for the present geometric configuration. In multi-tube arrangement, rate of melting in the lower region is augmented. Multi-tube LHSU shows 20% faster melting rate compared to single concentric tube LHSU. Solidification rate is however not found to be augmented by the intensification methods analysed here because of conduction dominant heat transfer.
Thermal characteristics of LHSU during both charging and discharging
process is established with installation of spiral fins. Spiral fins made of aluminium are brazed to the outer wall of the HTF pipe with geometrical configurations (chosen in such a way that thermocouples are accommodated at the same axial planes. With the installation of the fins, the rate of heat transfer via conduction mode is enriched in the radial direction along the entire length of the LHSU. This facilitates faster charging and discharging and reduces the energy storage and retrieval time. The heat transfer via convection is found to be restricted by fins in orientations where circulation of the melt flow is superior in the upper axial region. However, this depletion in the global convection is compensated by conduction through the fins and local convection within the fin spacing. With the installation of the spiral fins, highest improvement in the charging time of 35.22% is observed when the test section is aligned vertical (θ = 90º). Horizontal (θ = 0º) and inclined (θ = 45º) test section offer 23.5% and 21.8% reduction in the charging time. Imparting fins reduces the solidification time by 19.3% and 17.2% for the vertical and horizontal LHSU respectively. However, orientation has negligible influence
on the increase in solidification rate. Highest improvement in total cycle of 27.27% is observed for vertical LHSU.
Phase change material (PCM) has the capability to absorb, store and release heat during phase transition in a narrow range of temperature. Because of this, when integrated with the building envelope, PCMs show excellent potential to prevent heat gain and reduce the cooling loads of buildings. However, effectiveness of PCM is highly dependent on dynamic climatic condition. Thus, selection of suitable PCM for a given climatic condition becomes crucial for building envelope integrated with PCM. Numerical analysis is carried out for estimating the thermal performance of building envelope integrated with PCM for cooling load reduction. The analysis is carried out for seven cities representing different climatic zones of India during the six months of summer from March to August. Six commercially available PCMs are considered for analysis. The most suitable PCM for each city is determined based on a newly proposed performance characteristic index (PCI). The performance characteristic index is decided based on optimum values of ceiling temperature variation, time lag and decrement factor and melting and solidification cycle of PCM. The present study recommends a range of PCMs for achieving significant cooling load reduction in buildings of all climatic zones of India.