J.V.C. Vargas

A comparative study of elliptical and circular sections in one- and two-row tubes and plate fin heat exchangers

In this work, a two-dimensional (2-D) heat transfer analysis is performed in one- and two-row tubes and plate fin heat exchangers (circular and elliptical sections), using experimentally determined heat transfer coefficients from a heat and mass transfer analogy. The temperature distribution on the fin and air free stream, and the fin efficiency are determined for heat exchangers, with eccentricity 0.5 and 0.65, as a function of the Reynolds number. For tubes and plate fin heat exchangers, new numerical results of fin efficiency for elliptical tubes are compared with published results for circular tubes. A relative fin efficiency gain of up to 18% is observed in the elliptical arrangement, as compared to the circular one. The efficiency gain, combined with the relative pressure drop reduction of up to 25% observed in previous studies (Brauer 1964; Bordalo and Saboya 1995) show the elliptical arrangement has the potential for a considerably better overall performance than the conventional circular arrangement.

Integrative Thermodynamic Optimization of the Crossflow Heat Exchanger for an Aircraft Environmental Control System

This paper documents the process of determining the internal geometric configuration of a component by optimizing the global performance of the installation that uses the component. The example chosen is the crossflow heat exchanger used in the environmental control system of a modern aircraft. The optimization of global performance is achieved by minimizing the total entropy generation rate of the installation. There are three degrees of freedom in the heat exchanger configuration (the length-to-width and height-to-width aspect ratios, and the separator plate spacing ratio), which is subjected to two global constraints: total component volume, and total wall material volume (or weight/ density) of wall material. Numerical results show how the optimal configuration responds to changes in specified external parameters such as volume, weight, Mach number, diffuser inlet cross-sectional area, and the pressure at which the cabin air is initially bled from the engine compressor, It is shown that the optimal configuration is robust and that major features such as the ratios of channel spacings and flow lengths are relatively insensitive to changes in some of the external parameters. It is also shown that the optimal heat exchanger geometry is insensitive to the thermodynamic irreversibility caused by discharging the used ram air into the ambient.

Thermodynamic optimization of the match between two streams with phase change

The optimum thermodynamic match between two streams at different temperatures is determined by maximizing the power generation (or minimizing the entropy generation) associated solely with the stream-to-stream interaction. Each stream experiences a change of phase. It is shown that the optimum is marked by an optimal ratio between the stream mass flow rates, and an optimal ratio between the two heat exchanger sizes when the total heat transfer area is fixed. The sensitivity of the optimum relative to the various physical parameters of the two-stream arrangement is documented systematically. The study shows that the optimum is “robust” relative to changes in several parameters such as the distribution of heat transfer coefficient along the hot-end heat exchanger, and the model used for the thermodynamic behavior of steam.

Life cycle assessment of biomass production in microalgae compact photobioreactors

This paper presents a life cycle assessment (LCA) of industrial scale microalgae biomass production in compact photobioreactor (PBR) systems (2 × 5 × 8 m) for supplying biofuel/electricity generation processes and synthesis of new materials. Other objectives are as follows: (i) to compare the impact of various raw materials, substances, and services; and (ii) to evaluate environment‐relevant aspects of the proposed system as compared to microalgae raceway ponds. The life cycle inventory assessment shows that (i) only atmospheric CO2 is used for PBR microalgae cultivation, whereas in raceway ponds, injection of CO2 from fossil origin is largely required to allow for microalgae growth; and (ii) the PBR daily production rate of dry biomass is currently at 1.5 kg m−3 day−1 for each PBR, which is 12.82 times larger than the reported average 0.117 kg m−3 day−1 raceway ponds production. It is found that in general the association of the effects of the production of steel, PVC, and the packaging contribute to more than 85% of the total impact in each analyzed category. Therefore, to achieve PBR biomass production impact reduction and sustainability, PVC and steel utilization need to be minimized, as well as packaging materials. Based on the PBR LCA results, that is, due to no CO2 injection from fossil origin and low area occupation, it is expected that high density production of truly renewable microalgae biomass could be obtained from PBR systems.

Thermodynamic optimization of heat-driven refrigerators in the transient regime

The present work introduces a transient endoreversible model of a heat-driven refrigeration plant, which is driven by a fuel-burning heater. The model consists of a combustion chamber with negligible heat loss to the ambient, a refrigerator with three finite-size heat exchangers, namely, the evaporator between the refrigeration load and refrigerant, the condenser between the refrigerant and the ambient, and the generator between the combustion chamber and the refrigerant, and finally the refrigerated space. The total thermal conductance of the three heat exchangers is fixed. A thermodynamic optimization of the absorption cycle is then performed, reporting the operating conditions for minimum time to reach a prescribed cold-space temperature, thus maximum refrigeration rate, specifically, the optimal mass fuel flow rate and the optimal way of allocating the thermal conductance inventory. Half of the total supply of thermal conductance has to be divided equally between the generator and evaporator and the o...The present work introduces a transient endoreversible model of a heat-driven refrigeration plant, which is driven by a fuel-burning heater. The model consists of a combustion chamber with negligible heat loss to the ambient, a refrigerator with three finite-size heat exchangers, namely, the evaporator between the refrigeration load and refrigerant, the condenser between the refrigerant and the ambient, and the generator between the combustion chamber and the refrigerant, and finally the refrigerated space. The total thermal conductance of the three heat exchangers is fixed. A thermodynamic optimization of the absorption cycle is then performed, reporting the operating conditions for minimum time to reach a prescribed cold-space temperature, thus maximum refrigeration rate, specifically, the optimal mass fuel flow rate and the optimal way of allocating the thermal conductance inventory. Half of the total supply of thermal conductance has to be divided equally between the generator and evaporator and the other half allocated to the condenser, for optimal operation. A narrow range of fuel flow rates lead to the minimum time to achieve a prescribed cold-space temperature, thus stressing the importance of the transient analysis. Appropriate dimensionless groups were identified and the generalized results are reported in dimensionless charts.

First and Second Law Thermodynamic Analysis of a Domestic Scale Trigeneration System

The potential use of small-scale trigeneration systems in domestic homes, especially for emergency use in the event of a hurricane or natural disaster where electricity and useful power are at a premium, is increasing. They have the ability to produce both useful thermal energy and electricity from a single source of fuel such as gasoline, natural gas, or other alternate fuel. However, small-scale systems present some technological challenges in order to achieve a significant increase in efficiency over conventional systems. This paper addresses the fundamental question of the splitting of a hot exhaust into two heat recovery heat exchangers that are part of a trigeneration system. We consider a system which produces electricity, refrigeration, and hot water by recovering waste energy from a reciprocating internal combustion engine. First and second law analyses were performed on the refrigerator and water heating heat exchangers and on the overall system. An optimal splitting of the available hot exhaust stream between the refrigerator and the hot water heat exchangers is identified. The thermodynamic optima is sharp and robust with respect to the variation of refrigerant and water inlet temperatures.

A numerical investigation of the resin flow front tracking applied to the RTM process

Resin Transfer Molding (RTM) is largely used for the manufacturing of high-quality composite components and the key stage during processing is the resin infiltration. The complete understanding of this phenomenon is of utmost importance for efficient mold construction and the fast production of high quality components. This paper investigates the resin flow phenomenon within the mold. A computational application was developed to track the resin flow-front position, which uses a finite volume method to determine the pressure field and a FAN (Flow Analysis Network) technique to track the flow front. The mass conservation problem observed with traditional FE-CV (Finite Element-Control Volume) methods is also investigated and the use of a finite volume method to minimize this inconsistency is proposed. Three proposed case studies are used to validate the methodology by direct comparison with analytical and a commercial software solutions. The results show that the proposed methodology is highly efficient to determine the resin flow front, showing an improvement regarding mass conservation across volumes.

An International Component to Capstone Senior Design Projects

This paper describes our first attempt to introduce an international component into the Capstone Senior Design. The main objective of this first experience is to expose students to a global working environment, where in addition to the complexity of team dynamics, they have to face challenges associated with the distance, language, schedules and curriculum differences. This is a realistic microcosm of how many engineers will have to operate during their careers. We have started with two international teams composed of students from the Department of Mechanical Engineering at the FAMU-FSU College of Engineering (FAMU-FSU) and (i) the Department of Mechanical Engineering at the Federal University of Parana (UFPR) in Brazil and (ii) the Department of Electrical Engineering at the Polytechnic University of Bucharest (PUB) in Romania. The paper discusses the team selection process, the communication channels, the funding strategies, and the positive and negative elements associated with this first experience of adding an international character to the teams

Optimal Ground Tube Length for Cooling of Electronics Shelters

This paper presents a theoretical, numerical, and experimental study to investigate the possibility of optimizing the configuration (geometry) of underground heat exchangers for maximum heat transfer. The first part of the study identifies a novel fundamental optimization principle for maximizing heat transfer between a tube and its surroundings, which is expected to be present in any buried tube heat exchanger design. The second part presents a practical application of the fundamental principle: a simplified physical model to determine the temperature field inside an electronics shelter that uses an earth-air heat exchanger and the soil as a heat sink. A volume elements methodology is employed to obtain a system of ordinary differential equations with time as the independent variable that combines principles of classical thermodynamics and heat transfer. This allows the computation of the temperature and relative humidity fields at every instant inside the shelter. The numerical results obtained with the proposed model are validated by means of direct comparison with experimental temperature and relative humidity measurements. It is shown that the tube length can be optimized such that the maximum temperature reached inside the shelter is minimal. The results also demonstrate the potential of the utilization of buried tubes for cooling electronic packages. Since accuracy and low computational time are combined, the model is shown to be efficient and could be used as a tool for simulation, design, and optimization of electronic packages cooled by underground heat exchangers.

Thermal Model for the AC Armature Winding of a High Temperature Superconductor Airborne Motor

This paper describes a preliminary study on a cooling concept for an airborne high performance synchronous motor that has a High Temperature Superconductor (HTS) field winding: whereas the rotor is actually an HTS DC field winding, the armature is an AC copper winding, mounted in an iron-less stator — a so-called “air winding”. The efforts aimed at prototyping a low weight/volume motor lead to a dedicated thermal design where an important role is played by the thermal management of the AC winding, which is the siege of intense power dissipation by Joule and variable magnetic field effects. The analysis reveals thermal constraints that are overlooked by the initial, first stage electromagnetic design and that need to be addressed. The thermal analysis reported here is based on equivalent, lumped thermal circuits: (a) a simplified circuit, aimed at delivering fast, design class results, that may be solved analytically; (b) more complex schemes aimed at assessing variable regimes, which are solved numerically by a circuit simulator. Both approaches are valuable, and complement each other in the quest for a meaningful preliminary design.© 2005 ASME