III. Resultados
III.1. Capítulo I. Estudio epidemiológico de Anisakis spp. en los peces del norte de
A compact latent heat storage module was developed to investigate the performance of PCMs in a rectangular container. The rectangular container was made from 3mm aluminum sheet bolted to four 37mm aluminum C profiles. A 15mm copper pipe heat exchanger was inserted within the container to exchange heat between the water flow and the PCM. The copper pipe comprised of 39 U-bends soldered to 511mm straight tube sections. Copper metal fins were soldered to the tube outer surface to improve heat transfer to the PCM to provide the charge illustrated in figure 6.16a and 6.16b. A simplified view of the experimental gravity–fed water test rig is presented in figure 6.16a.
(a) experimental rig assembled (b) PCM slab section view
Figure 6.16: A schematic diagram of the experimental test rig used with the dis- position of the thermocouples employed and simplified diagram demonstrating the heat balances of the thermal store tested.
The rectangular storage contained 18.3L of RT44HC [20], theoretically able to store 1.39kW hthwhen heated/cycled from 20 to 70◦C. The thermal store key dimen- sions and main characteristics are presented in table 6.6. Water was used as the heat transfer fluid, while the PCM filled the remaining volume around the tube within the rectangular container. The PCM rectangular container was inserted within a wooden enclosure 12mm thick MDF board (thermal conductivity of 0.101W/m.K) with 20mm air gaps between the container sides and the PCM rectangular enclosure. Adding 5 copper fins spaced 32mm apart soldered transversally to the copper pipe increased the total heat transfer area within the PCM by 52%.
Table 6.6: Thermal store key dimensions, heat transfer fluid flow and storage ma- terial properties for the experimental work performed.
PCM store width [mm] 600 Number of fins 5
PCM store length [mm] 950 PCM fraction [%] 89
PCM store thickness [mm] 37 Specific HT area [m2/m3
P CM] 48
Coil length [m] 12 Charging/Discharging error [+ − %] 2.37/2.55 Tube OD (ID) [mm] 15.86 (12.45) Store volume [m33] 0 PCM (RT44HC) PCM volume [m3] 0 Hsl (37-46◦C) [kW h/m3] 50 Cps (20-37◦C) [kW h/m3.K] 0.469+0.0045*T HTF(average values): Cpl (46-70◦C) [kW h/m3.K] 0.368+0.0027*T Flow rate [g/s] 143 λs(20-37◦C) [W/m.K] 0.450-0.00096*T Re 25 504 λl (46-70◦C) [W/m.K] 0.188-0.00036*T Pr 4 β [1/K] × 10−4 8 Nu 155 ζ [m2/s] × 10−6 9 hcv [W/m2.K] 7399 Pr 150-1.333*T
A Huber Unistat Tango was used to indirectly exchange heat with the gravity-fed water loop, using a programmable proportional-integral-derivative (PID) controller to maintain the set point temperature at constant values during the store charg- ing and discharging. Thermocouples were fixed at the HTF inlet and outlet of the thermal store. The PCM temperature was monitored using thermocouples located at the 2 positions illustrated in figure 6.16a, and the flow rate was monitored by a turbine flow meter from Icenta, its signal converted to current and recorded in a compact DAQ card from National Instruments. The uncertainty of the turbine flowmeter reading is +-0.15% quoted in the supplier data sheet and the thermocou- ples uncertainty was already determined and reported in Chapter 5 (+-0.01◦C).
Figure 6.17: Comparison between experimental and predicted measurements for the inlet/outlet water temperatures and PCM store temperature.
The experiment was carried out in two modes: charging and discharging; and the data presented in figure 6.17 and figure 6.18 refers only to the 5th cycle. In the charging mode, the initial state of the PCM was solid at room temperature of
20.3◦C. The HTF was heated to the required temperature by the Huber tango heat exchanger and pumped through the copper pipe. The charging was completed when all the PCM in the tank had melted fully and its temperature stabilized around 70◦C and temperature readings were taken every minute. In the discharging mode, the HTF temperature was reduced to 20◦C and temperature readings continued until the PCM temperature had stabilized around 20◦C. Figure 6.17 presents the exper- imental temperature measurements and model predictions obtained for the water flow inlet/outlet and PCM store during the charging and discharging processes. The PCM had fully melted at 180 minutes, as can be seen in figure 6.17.
Figure 6.18 presents the experimental measurements and model predictions for the heat transfer rates and stored energy. The heat transfer rate was constant at around 800W between t=30 and t=80 minutes during charging. During the discharge a lower rate of around 700W occurred between t=360 and t=460 minutes. In the model, neglecting buoyancy driven convection currents in the molten PCM led to a slower change in temperature the charging cycle, as shown in figure 6.18. This led to a lower charging rate and consequently less energy charged.
Figure 6.18: Experimental measurements and model predicted profiles of the thermal store for the heat transfer rate and storage capacity.
To improve the model accuracy for the melting process, an equivalent thermal conductivity increase to account for convection in the molten PCM was calculated based on the Rayleigh number of the temperature difference between the melting point of the PCM and the actual temperature of the PCM, expressed in equation 6.14.
Figure 6.19: Schematic diagram of the discretization between the transversal fins used to enhance thermal conductivity within the PCM.
Based on [161], the Nusselt number was then calculated using correlations for free convection on a vertical annulus [161], displayed in equation 6.15, and applied to each PCM node. The annulus height (Hf in) considered was the spacing between fins and the annulus thickness (s) varied according to the fraction of molten PCM.
Rai,j = g · β · s3 i,j· (Ti,j − Tm ζ2 · P ri,j (6.14) N uRai,j = 0, 49 · Rai,j· H f in si,j 2 862 ·Hf in si,j 4 ·rtube Hf in + Rai,j H f in si,j 30.95 ·rtube Hf in 0.8 · P ri,j (6.15)
It can be seen from figure 6.17 and 6.18 that conductivity enhancement in the PCM led to a more accurate prediction of the melting process of the PCM, pre- dicting the water flow outlet temperature more accurately when compared with the experimental measurements.
Observing the discharge profiles of figure 6.18, it can be seen that the conduc- tivity enhancement in the PCM has negligible effect on the water flow inlet/outlet temperature difference, mainly because convection in the melt is suppressed dur- ing the freezing process, and both model predictions agree with the experimental results.