Study on heat transfer behavior of high Prandtl number fluid and its feasibility to passive heat. The reliable assessment of the heat transfer performance therefore requires the consideration of property variation. In the previous studies of the same research group, the distinct heat transfer behavior of high Prandtl number oil in the natural circulation system was reported.
This study extends the experimental work to the forced convective heat transfer and discusses the unique feature of heat transfer behavior of high Prandtl number oil. Specifically, the convective heat transfer performance of high Prandtl number oil in the transitional flow regime is not fully understood with the previous correlations. The theoretical discussion of the local natural convection of high Prandtl number oil verifies its existence in the heat transfer between liquid and heated wall.
Research background and motivation
Review on Heat transport system using High-Pr fluid
The commercial high-efficiency solar power facilities, especially parabolic trough CSPs, use both synthetic oil and molten salt as heat transfer fluids for their systems 18. The thermal oils have a wide operating temperature range with a high heat capacity that has advantages in heat transfer and heat storage systems. HITEC is also widely used as a heat transfer fluid and thermal storage medium for its low melting point, good thermal stability, similar viscosity to water, and comparable thermal conductivity 24,25 .
The ideal properties of heat transfer fluid for closed heat transfer systems, especially for commercial facilities, include high operating temperature with a low melting point, thermal stability, low vapor pressure, material compatibility, low viscosity, high thermal conductivity, high heat capacity and economic properties. efficiency 26.27. In this sense, the heat transfer properties of molten salts are not superior to those of other heat transfer fluids, since molten salts have a similar heat capacity, but low thermal conductivity and high viscosity 28. In other words, the heat transfer performance of molten salts in the heat transport system should mainly are assessed with respect to thermophysical properties such as heat capacity, thermal conductivity and fluid viscosity.
Review on Heat Transfer behavior of High-Pr fluid
On the other hand, Fand 36 proposed additive form of Nusselt correlation with respect to Rayleigh number and Prandtl number. As shown in Table 1.1 and Table 1.2, the majority of Nusselt correlations are only applicable to turbulent flow region where Reynolds number is greater than 10,000. As mentioned in the previous section, the typical energy or heat transport system is closed circulation loop, mainly made by circular tube.
In that aspect, there are always doubts about the feasibility of classical Nusselt correlations with the evaluation of heat transfer performance in practical heat transport system. Especially for high Prandtl number fluids such as oils and salts, the reliability in using the classical Nusselt correlations is weakened, as the classical correlations still do not take into account the distinct heat transfer characteristic of high Prandtl number fluid. Thus, the present study reports the distinct heat transfer characteristic of high Prandtl number fluid and suggests simple correlation specified to high Prandtl number fluids.
Research Objectives and Scope
FEASIBILITY TEST ON PASSIVE HEAT TRANSPORT SYSTEM USING HIGH-
Development of adjoint sensitivity model
- Background of adjoint sensitivity model
- Adjoint sensitivity model for passive heat transport system…
- Validation of adjoint sensitivity model for passive heat transport system…
Thus, the additive method requires solving a single forward equation and a single additive equation to complete the sensitivity calculation, regardless of the number of parameters. The additional sensitivity equations were developed for the engineering problem with the objective functionals of mass flow and temperature. The additional sensitivity equations were developed for the engineering problem with the objective functionals of mass flow and temperature.
Finally, the adjoint equations were defined to obtain the last integral term in Eq. to eliminate. and the sensitivity of the objective functional was expressed as:. where the first and second terms became zero by imposing the initial and boundary conditions of adjacent sensitivity equations, respectively. The numerical scheme for the sensitivity analysis used implicit Euler discretization, which allowed to preserve the properties of the adjoint equations 62. The computation time of direct and adjoint methods with the number of design parameters. a) Time evolution of the adjacent mass flux from Eq. 32) (b) 3D surface of the adjacent temperature from Eq.
Assessment results of system feasibility using high-Prandtl number fluid
- Sensitivity test on passive heat transport system using high-Pr fluid…
- Feasibility test on high-Pr fluid in passive heat transport system
- Reliability assessment of Nusselt correlation for high-Pr fluid
Moreover, the sensitivities of mass flow with respect to parameters along with the temperature, such as href and Tw, showed the greatest increase up to 130 % among all parameters. The sensitivities of Nusselt number also showed significant increase in respect of all the parameters except for floc which had no direct relationship with temperature. The comparison between cases showed that the absolute sensitivity of mass flux with respect to the performance parameters of heat exchanger such as heat transfer coefficient and wall temperature increased significantly for the test case 2.
In addition, if the orientation of the heat exchanger must be changed, the sensitivity of the mass flow in relation to the performance parameters of the heat exchangers must first be tested. Temperature sensitivity also differed at the low reference temperature, with respect to most parameters. Unlikely for mass flux sensitivity change, temperature sensitivity in relation to fluid properties and major friction factors also showed significant variation.
2.22(a)-(d) show the sensitivities of the Nusselt number to the fluid properties as the maximum operating temperature varies, for 4 different fluids. For all analyzed fluid cases, the sensitivity of the Nusselt number to the fluid properties increased slightly at high operating temperature. To make the analyzes precise, only sensitivities to the empirical constants in the classical Nusselt correlation were presented in this section.
Subsequently, the larger sensitivities for high Pr fluids, especially with respect to constants associated with the Pr term, increased the uncertainties in the Nusselt number prediction using existing correlations. It was clear that the sensitivities of the Nusselt number with respect to exponent constants associated with the Prandtl number were dependent on the fluids. However, the sensitivities of high Pr fluids were largest with respect to c2 and second largest with respect to b2.
Absolute normalized sensitivity of the Nusselt number for different geometrical conditions of the heat exchanger (*Red line indicates a negative value). Normalized sensitivity of Nusselt number to fluid properties as operating temperature increases for different fluids: (a) liquid sodium (b) water (c) DOWTHERM. Normalized sensitivity of Nusselt number with respect to empirical constants in a typical Nusselt correlation (Normalized sensitivity of water is set as Ref.).
HEAT TRANSFER BEHAVIOR OF HIGH-PRANDTL NUMBER FLUID
Experimental Setup and Procedures
- Experimental facility
- Test procedure and experimental uncertainty
The experimental system for the heat transfer performance of high Prandtl number fluid mainly consisted of a heated section, water-cooled heat exchanger, pump, expansion pipe and collection system, as illustrated in Fig. The entire experimental system was a rectangular closed circuit with a total height of 2.66m and a width of 0.6m. The vertical heated section was wrapped by the electrical resistive coil wire generating heat source and the tube surface throughout the loop was insulated using fiberglass insulator to minimize heat loss.
The maximum power input was limited to 1kW to prevent boiling of the DOWTHERM RP working fluid. In the upper part of the right vertical side, a tube-to-tube heat exchanger was installed, cooled by water with constant temperature and mass flow. The expansion tube located at the highest altitude allowed thermal expansion of the liquid.
The turbine pump with the frequency converter, installed in the lower part, made it possible to conduct the experiments within the Reynolds number range from 2000 to 10,000. The wall temperatures in the heater were measured by K-type thermocouples placed on the outer surface of the tube. The bulk fluid temperatures in the heater and cooler were also measured by four K-type thermocouples, the tips of which were placed in the radially central position in the tube.
The volumetric flow rate of DOWTHERM RP fluid was measured using the turbine flow meter downstream of the cooling section. Here we defined the Grashof number of fluid using the radial temperature difference, ∆Tm to consider the radial natural convection by buoyancy effect. According to the current experimental conditions, uncertainties in Reynolds number, Prandtl number, Grashof number and Nusselt number were respectively and 0.2%.
Experimental results
- Forced heat transfer behavior of high-Pr oil
- Discussion on heat transfer behavior of high-Pr fluid
Instead, natural convective heat transfer has a significant role in the heat transfer between the fluid and the hot wall. Especially, the fluid with high Pr number caused large temperature gradient near the wall, which enhanced the local natural convective heat transfer. Thus, it is worth discussing the local enhanced natural convective heat transfer as a distinct feature of high Pr fluid heat transfer.
In other words, the influence of high-Pr on the average and local convective heat transfer is highly questionable. The forced convective heat transfer of molten salts in circular tubes was investigated and associated correlations were proposed from the experimental studies. In the 21st century, many researchers have investigated the convective heat transfer of molten salt in different types of fluid path.
95,96 designed transversely corrugated tubes to study molten salt heat transfer, while Lu et al. In addition, it was proven that HITEC could reproduce the natural heat transfer of fluorine-based molten salts. Furthermore, the correlations listed were essentially applicable to the heat transfer of the external flow.
In addition, the fundamental insight into the distinctive heat transfer behavior of high-Pr fluid was introduced and roughly tested. However, the fundamental knowledge about the single-phase heat transfer of molten salt is still insufficient to design a heat transport system and predict its performance. Finally, the idea of heat transfer from high-Pr fluids can provide a physical basis for understanding the thermal-hydraulic properties of molten salts.
Study of flow characteristics of high-Pr heat transfer fluid near the wall in a rectangular natural circulation loop. Study on natural circulation heat transfer and flow characteristics of high-pr oil simulant of molten salts. Convective heat transfer in the laminar-turbulent transition region with molten salt in a circular tube.
