A finite element analysis was undertaken to determine the effects of the following factors on the heat losses through residential floor slabs: outdoor air temperature, deep soil temperature, thermal conductivity of the underlying soil, and the configuration of any thermal insulation used. The study confirmed that the losses occur primarily near the edge of the slab and are proportional to the product of slab perimeter and the indoor/outdoor temperature difference, a relationship familiar to the HVAC community. However, the constant of proportionality was found to be strongly dependent on both the insulation configuration and the soil thermal conductivity. The latter dependency has seemingly been ignored in past studies. Thus, accurate predictions of slab heat losses must include considerations of the soil underlying the slab.
A closed loop cooling system that uses the earth as a heat sink to dissipate heat for the energy system’s thermal management is described. The proposed cooling approach employs a concentric tube heat exchanger situated above ground to transfer heat from the system (e.g., power plant condenser) to a separate cooling water loop buried at a specified depth below ground. A parametric study was performed to evaluate the efficacy of the thermal management potential of ground-coupled systems in industrial applications. It revealed that such a condenser design is generally capable of dissipating less than 1.5 MW of heat. A mathematical model is developed to size the required piping for different systems.
This paper reports a numerical design analysis of electrostatically actuated micromembranes. We systematically compare membrane performance in terms of natural frequencies, pull-in voltage (the bias voltage at which the membrane contacts the base electrode) and the effects of variable leg lengths for a given membrane size. Some experimental data on membrane deflection profiles versus bias voltage is included along with some experimentally determined pull-in voltages. Polysilicon micromembranes were successfully fabricated using the low cost MUMPs process that limits the user to three structural layers. The devices are designed with an emphasis on the response of the membrane to applied DC bias voltage to allow for variable stiffening. Circular membranes with diameters ranging from 60 to 160 μm, suspended 2 μm over square back plates of side lengths varying from 60 to 140 μm are investigated for voltages up to 90 volts. Three-dimensional electromechanical finite element simulations have been performed. Pull-in voltage values from simulations compare favorably with the measured results. It was observed that, for maximum deflection of the membrane upon application of DC bias voltage, the optimal dimensions for back plate and top membrane should fall within the ranges 80–120 μm and 80–140 μm, respectively.
In this article, first, graphene oxide nanosheets were synthesized in-house according to the modified Hummers method, and these nanosheets were used to prepare graphene oxide nanofluids at two concentrations. Then the thermophysical properties of nanofluids were characterized using X-ray diffraction analysis, a scanning electron microscope, and UV–Vis spectrophotometry. The particle size distribution was investigated using dynamic light scattering. Then, a fundamental study was conducted on the thermal-hydraulic characteristics of graphene oxide nanofluids flowing through a straight copper tube. An experimental setup was developed to find the heat transfer characteristics and pressure drop of nanofluids in the test section consisting of a copper tube with constant heat flux. The flow regimes and associated pressure drop and heat transfer characteristics at varying flow rate were investigated at three different heat flux conditions of 7.4, 9.1, and 12.6 kW/m2. Due to the increase in viscosity, flowrate and Reynolds number decreased from 0.01 to 0.1 wt% of graphene oxide nanofluids at constant pump frequency. Experimental data obtained for water were validated with the findings from the literature, and the correlations were formulated for the Nusselt number and Reynolds number by considering the multiple regression analysis. The convective heat transfer coefficient for graphene oxide at 0.01 wt% was higher when compared to graphene oxide at 0.1 wt% and water. The variation of Nusselt number with the heat flux and velocity was insignificant.
An organic rankine cycle operates under the same principle as a steam rankine cycle, but with a lower operating temperature and pressure. These operating conditions are a result of substituting, into the closed loop system, a working fluid other than water. This allows a lower grade heat to act as a fuel for operation. The organic rankine cycle can be used in conjunction with a steam rankine cycle to recapture waste heat and improve overall system efficiency. A study was conducted in order to find feasible waste heat recovery applications and the industries which would benefit most from those applications. This study shows calculations and quantitative results for theoretical organic rankine cycle operation. These calculations include energy generation of the system at variable waste heat temperatures. Additionally, economic cost calculations are supplied in order to demonstrate the simple payback period for various system sizes. Two potential applications are reviewed, demonstrating the need for year round operation. Furthermore, current technologies are evaluated to demonstrate the viability of organic rankine cycles in industries with reliable lowgrade waste heat. Several examples of plug and play models are listed along with a variety of other models. Some of these plug and play models help emphasize the fact that implementation is not very complex and could easily be adopted. Using numerical analysis, backed by several case studies, it is determined that an organic rankine cycle can be a useful and economical means of waste heat recovery.
A dynamic modeling study and field validation of a cooling tower serving a chiller are performed in this study to evaluate the efficiency of the cooling tower under various conditions. The dynamic model examines the losses and thermal capability of the cooling tower. The cooling tower used in this study for field validation is located at Cookeville, Tennessee, USA. The whole setup is fully instrumented to record the water and air flow rates and temperature; make-up water and blow down water flow rates; power consumption of pump; fan and chiller; weather data; etc. with one-hour resolution. The outcomes of the model are validated with the recorded data of the cooling tower and the test condition data. Furthermore, the effects of using Variable Frequency Drive (VFD) on the cooling tower’s fan power consumption are investigated. Additional recommendations on improving the energy efficiency and reducing the water losses are suggested based on the modeling data.
