Potential for Passive Radiative Cooling by PDMS Selective Emitters

The scalability and implementation of selective emitters in passive radiative cooling applications are limited by the high fabrication costs due to the complexity of these structures. The usage of commercially available polymers in selective emitters holds potential in lowering the cost of radiative cooling solutions. In this work, we demonstrate that thin films of polydimethyl-siloxane (PDMS) on aluminum substrates act as radiative coolers by selectively emitting in the wavelength range of 8 μm to 13 μm, where the Earth’s atmosphere is highly transparent. We also show that our device can achieve passive cooling up to 12 °C below the ambient temperature under the night sky. This suggests that PDMS, especially due to its ease of deposition, may be a viable selective emitter in passive radiative cooling applications.

Experimental Study of Condensation Flow Inside Horizontal Smooth, Herringbone and Three Enhanced Surface Tubes

Enhanced condensation heat transfer of two-phase flow on the horizontal tube side receives more and more concerns for its fundamentality and importance. Experimental investigations on convective condensation were performed respectively in different horizontal tubes: (i) a smooth tube (11.43 mm, inner diameter); (ii) a herringbone tube (11.43 mm, fin root diameter); and (iii) three enhanced surface (EHT) tubes (11.5 mm, equivalent inner diameter): 1EHT tube, 2EHT-1 tube and 2EHT-2 tubes. The surface of EHT tubes is enhanced by arrays of dimples with the background of petal arrays. Experiments were conducted at a saturation temperature of approximately 320 K; 0.8 inlet quality; and 0.2 outlet quality; 72–181 kg·m−2·s−1 mass flux using R22, R32 and R410A as the working fluid. The refrigerant R32 presents great heat transfer performance than R410A and R22 at low mass flux due to its higher latent heat of vaporization and larger thermal conductivity. The heat enhancement ratio of the herringbone tube is 2.72–2.82, rated number one. The primary dimples on the EHT tube increase turbulence and flow separation, and the secondary petal pattern produce boundary layer disruption to many smaller scale eddies. The 2EHT tubes are inferior to the 1EHT tube. A performance factor is used to evaluate the enhancement effect except of the contribution of area increase.

Optimal Position of Rectangular Vortex Generator for Heat Transfer Enhancement in a Flat-Plate Channel

Heat transfer is a naturally occurring phenomenon which can be greatly enhanced by introducing longitudinal vortex generators (VGs). As the longitudinal vortices can potentially enhance heat transfer with small pressure loss penalty, VGs are widely used to enhance the heat transfer of flat-plate type heat exchangers. However, there are few researches which deal with its thermal optimization. Three dimensional numerical simulations are performed to study the effect of angle of attack and attach angle (angle between VG and wall) of vortex generator on the fluid flow and heat transfer characteristics of a flat-plate channel. The flow is assumed as steady state, incompressible and laminar within the range of studied Reynolds numbers (Re = 380, 760, 1140). In the present work, the average and local Nusselt number and pressure drop are investigated for Rectangular vortex generator (RVG) with varying angle of attack and attach angle. The numerical results indicate that the heat transfer and pressure drop increases with increasing the angle of attack to a certain range and then decreases with increasing angle of attack. Moreover, the attach angle also plays an importance role; a 90° attach angle is not necessary for enhancing the heat transfer. Usually, heat transfer enhancement is achieved at the expense of pressure drop penalty. To find the optimal position of vortex generator to obtain maximum heat transfer and minimum pressure drop, the data obtained from numerical simulations are used to train a BRANN (Bayesian-regularized artificial neural network). This in turn is used to drive multi-objective genetic algorithm (MOGA) to find the optimal parameters of VGs in the form of Pareto front. The optimal values of these parameters are finally presented.

Monte Carlo Simulation for Radiative Transfer in a High-Pressure Industrial Gas Turbine Combustion Chamber

Radiative heat transfer is studied numerically for reacting swirling flow in an industrial gas turbine burner operating at a pressure of 15 bar. The reacting field characteristics are computed by Reynolds-averaged Navier-Stokes (RANS) equations using the k-ε model with the partially stirred reactor (PaSR) combustion model. The GRI-Mech 2.11 mechanism, which includes nitrogen chemistry, is used to demonstrate the the ability of reducing NOx emissions of the combustion system. A Photon Monte Carlo (PMC) method coupled with a line-by-line spectral model is employed to accurately account for the radiation effects. CO2, H2O and CO are assumed to be the only radiatively participating species and wall radiation is considered as well. Optically thin and PMC-gray models are also employed to show the differences between the simplest radiative calculation models and the most accurate radiative calculation model, i.e., PMC-LBL, for the gas turbine burner. It was found that radiation does not significantly alter the temperature level as well as CO2 and H2O concentrations. However, it has significant impacts on the NOx levels at downstream locations.

Characterization of an Air-PCM Energy Storage Design for Air Handling Unit Applications

This paper will discuss the characterization of an air-PCM storage design for commercial air handling unit (AHU) applications during winter. The air-PCM storage design consists of two rows of 29 aluminum flat plate containers (0.45 m × 0.35 m × 0.01 m) filled with PCM, vertically aligned leaving an air channel between each plate of 0.011 m wide. The storage device was placed within a closed air loop which conditions the air to the desired testing temperature and velocity. The PCM selected for testing was RT44HC with a melting temperature of 44 °C. This PCM was chosen for its similar properties to other PCMs having lower melting temperatures (in the range of 5 to 18°C) that could be used in actual HVAC application implementation. The system was instrumented and calibrated with Type T thermocouples and a velocity sensor. The system was tested at various inlet temperatures (55°C to 63°C for charging and 12°C to 25°C for discharging) and flow rates. The instantaneous heat transfer rates and total energy storage were calculated for each test from the data collected. The results provide a baseline value for heat transfer rates in a simple air-PCM design, to be used for model validation.

Improved Melting and Solidification in Thermal Energy Storage Through Topology Optimization of Highly Conductive Fins

Thermal energy storage units based on phase change materials (PCMs) need a fine design of highly conductive fins to improve the average heat transfer rate. In this paper, we seek the optimal distribution of a highly conductive material embedded in a PCM through a density-based topology optimization method. The phase change problem is solved through an enthalpy-porosity model, which accounts for natural convection in the fluid. Results show fundamental differences in the optimized layout between the solidification and the melting case. Fins optimized for solidification show a quasi-periodic pattern along the angular direction. On the other hand, fins optimized for melting elongate mostly in the bottom part of the unit leaving only two short baffles at the top. In both cases, the optimized structures show non-intuitive details which could not be obtained neglecting fluid flow. These additional features reduce the solidification and melting time by 11 % and 27 % respectively compared to a structure optimized for diffusion.

Design, Fabrication and Testing of Cooling Systems for a Particle Accelerator

This paper discusses the design, analysis, and testing of a Water Cooling System (WCS) for a Drift Tube Linear (DTL) Particle Accelerator structure at the Los Alamos Neutron Science Center (LANSCE). The DTL WCS removes large amounts of dissipated electrical energy in a very controlled manner to maintain a constant temperature of the large structure. First, the design concept and method of water temperature control is discussed. Second, the layout of the water cooling system, including the selection of plumbing components and instrumentation is presented. Next, the development of a numerical nodal network model, used to size the plumbing, pump, control valves, and mixing tank (heat exchanger), is discussed. Finally, empirical pressure, flow rate, and temperature data from a functioning DTL water cooling system are used to assess the water cooling system performance and validate the numerical model.

Performance of Mixing Ventilation System Coupled With Dynamic Personalized Ventilator for Thermal Comfort

This study optimizes the performance of a mixing ventilation system coupled with a personalized ventilator that emits a cool sinusoidal horizontal airflow jet towards the occupant upper body in order to achieve good overall thermal comfort and good air quality in the occupant breathing zone. A transient 3-D computational fluid dynamics (CFD) model coupled with a transient bio-heat model was deployed to predict airflow and temperature fields in the space and around the occupant as well as segmental skin temperature profiles for local and overall thermal sensation and comfort analysis. Simulations were performed using the CFD model to determine the airflow optimal supply frequency, mean flow rate and amplitude at room temperature of 25 °C and PV jet temperature of 22 °C. The system also showed, that when increasing frequency at fixed mean flow rate, thermal comfort increased from by 15.2 %. However when increasing mean flow rate at a fixed frequency, thermal comfort dropped at the low frequency of 0.3 Hz but remained acceptable at the higher frequency of 0.5 Hz.

Computational Study of Flow and Heat Transfer With Anti Cross-Flows (ACF) Jet Impingement Cooling for Different Heights of Corrugate

In the present study, a flow visualization and heat transfer investigation is carried out computationally on a flat plate with 10×1 array of impinging jets from a corrugated plate. This corrugated structure is an Anti-Cross Flow (ACF) technique which is proved to nullify the negative effects of cross-flow thus enhancing the overall cooling performance. Governing equations are solved using k-ω Shear Stress Transport (SST) turbulence model in commercial code FLUENT. The parameter variation considered for the present study are (i) three different heights of ACF corrugate (C/D = 1, 2 & 3) and (ii) two different jet-to-target plate spacing (H/D = 1 & 2). The dependence of ACF structure performance on the corrugate height (C/D) and the flow structure has been discussed in detail, therefore choosing an optimum corrugate height and visualizing the three-dimensional flow phenomena are the main objectives of the present study. The three-dimensional flow separation and heat transfer characteristics are explained with the help of skin friction lines, upwash fountains, wall eddies, counter-rotating vortex pair (CRVP), and plots of Nusselt number. It is found that the heat transfer performance is high at larger corrugate heights for both the jet-to-plate spacing. Moreover, the deterioration of the skin friction pattern corresponding to the far downstream impingement zones is greatly reduced with ACF structure, retaining more uniform heat transfer pattern even at low H/D values where the crossflow effects are more dominant in case of the conventional cooling structure. In comparison of the overall heat transfer performance the difference between C/D = 3 & C/D = 2 for H/D = 2 is significantly less, thus making the later as the optimal configuration in terms of reduced channel height.

Enhanced Film Cooling Effectiveness by Integration of Horn-Shaped and Cylindrical Holes

In search of improved cooling of gas turbine blades, the thermal performances of two different film cooling hole geometries (horn-shaped and cylindrical) are investigated in this numerical study. The horn-shaped hole is designed from a cylindrical hole by expanding the hole in the transverse direction to double the hole size at the exit. The two hole shapes are evaluated singly and in tandem. The tandem geometry assumes three configurations made by locating the cylindrical hole at three different positions relative to the horn-shaped hole such that their two axes remain parallel to one another. One has the cylindrical hole downstream from the center of the horn-shaped hole, a second has the cylindrical hole to the left of (as seen by the flow emerging from the horn-shaped hole) and at the same streamwise location as the horn-shaped hole (θ = 90°) and the third has an intermediate geometry between those two geometries (downstream and to the left of the horn-shaped hole - θ = 45°). It is shown from the simulation results that the cooling effectiveness values for the θ = 45° and 90° cases are much better than that for θ = 0° (the first case), and the configuration with θ = 45° exhibits the best cooling performance of the three tandem arrangements. These improvements are attributed to the interaction of vortices from the two different holes, which weakens the counter-rotating vortex pairs inherent to film cooling jet to freestream interaction, counteracts with the lift forces, enhances transverse tensile forces and, thus, enlarges the film coverage zone by widening the flow attachment region. Overall, this research reveals that integration of horn-shaped and cylindrical holes provides much better film cooling effectiveness than cases where two cylindrical film cooling holes are applied with the same tandem configuration.