J. C. Ordonez

Entropy generation minimization in parallel‐plates counterflow heat exchangers

This paper shows that the main architectural features of a counterflow heat exchanger can be determined based on thermodynamic optimization subject to volume constraint. It is assumed that the channels are formed by parallel plates, the two fluids are ideal gases, and the flow is fully developed, laminar or turbulent. In the first part of the paper, it is shown that the irreversibility of the heat exchanger core is minimized with respect to (1) the ratio of the two-channel spacings, and (2) the total heat transfer area between the two streams. In the second part, the entropy generation rate also accounts for the irreversibility due to discharging the spent hot stream into the ambient. It is shown that the design can be optimized with respect to (1), (2) and (3) the ratio of the capacity rates of the two streams. The optimized features of the geometry are robust with respect to whether the external discharge irreversibility is included in the entropy generation rate calculation. Copyright

Designed porous media: Optimally nonuniform flow structures connecting one point with more points☆

Abstract This paper shows analytically and numerically how an originally uniform flow structure transforms itself into a nonuniform one when the objective is to minimize global flow losses. The flow connects one point (source, sink) to a number of points (sinks, sources) distributed uniformly over a two-dimensional domain. In the first part of the paper, the flow between neighboring points is modeled as fully developed through round tubes. It is shown that flow ‘maldistribution’ and the abandonment of symmetry are necessary for the development of flow structures with minimal resistance. The search for better flow structures can be accelerated: tubes that show a tendency of shrinking during the search can be assumed absent in future steps of structure optimization. In the second part of the paper, the flow medium is continuous and permeated by Darcy flow. The development of flow structures (channels) is modeled as a mechanism of erosion, where elements of the original medium are removed one by one, and are replaced with a more permeable medium. The elements selected for removal are identified based on two criteria: maximum pressure integrated over the element boundary, and maximum pressure gradient. The flow structures generated based on the pressure gradient criterion have consistently smaller flow resistances. As flow systems become smaller and more compact, the flow systems themselves become “designed porous media”. These design optimization trends revealed are generally applicable in constructal design, i.e., where miniaturization, global performance, compactness and complexity rule the design.

Minimum power requirement for environmental control of aircraft

This paper addresses two basic issues in the thermodynamic optimization of environmental control systems (ECS) for aircraft: realistic limits for the minimal power requirement, and design features that facilitate operation at minimal power consumption. Four models are proposed and optimized. In the first, the ECS operates reversibly, the air stream in the cabin is mixed to one temperature, and the cabin experiences heat transfer with the ambient, across its insulation. The cabin temperature is fixed. In the second model, the fixed cabin temperature is assigned to the internal solid surfaces of the cabin, and a thermal resistance separates these surfaces from the air mixed in the cabin. In the third model, the ECS operates irreversibly, based on the bootstrap air cycle. The fourth model combines the ECS features of the third model with the cabin-environment interaction features of the second model. It is shown that in all models the temperature of the air stream that the ECS delivers to the cabin can be optimized for operation at minimal power. The effect of other design parameters and flying conditions is documented. The optimized air delivery temperature is relatively insensitive to the complexity of the model; for example, it is insensitive to the size of the heat exchanger used in the bootstrap air cycle. This study adds to the view that robustness is a characteristic of optimized complex flow systems, and that thermodynamic optimization results can be used for orientation in the pursuit of more complex and realistic designs.

Power extraction from a hot stream in the presence of phase change

This paper considers the basic thermodynamic optimization problem of extracting the most power from a stream of hot exhaust when the contact heat transfer area is fixed. It shows that when the receiving (cold) stream boils in the counterflow heat exchanger, the thermodynamic optimization consists of locating the optimal capacity rate of the cold stream. At the optimum, the cold side of the heat transfer surface divides itself into three sections: liquid preheating, boiling and vapor superheating. Numerical results are developed for a range of design parameters of applications with either water or toluene on the cold side. It is shown that the optimal design is robust, because several of the design parameters have only a weak effect on the optimal design.

Constructal dendritic geometry and the existence of asymmetric bifurcation

This paper considers the fundamental question of whether the optimal geometry of dendritic tree networks is symmetric or asymmetric. Asymmetry of dendritic networks is a result of the relation between flow resistance and flow fraction at each bifurcation node. Asymmetry of bifurcation (asymmetry of Y-shaped assemblies) appears when the flow fraction at each bifurcation node is not equal to one-half. Asymmetric bifurcation provides lower flow resistance than symmetric bifurcation. Murray’s law of bifurcation (Di+1∕Di=2−1∕3) is valid only when the flow fraction at every bifurcation node in dendritic networks is one-half and the networks are symmetric. General rules to construct asymmetric trees are developed and reported in this paper. It is shown that even through pressure drops across round and square cross-sectional shape channels are different; their flow resistances can be expressed by similar relations.

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.

Electro-Thermal Model for HTS Motor Design

Superconductivity could become an enabling technology for all-electric aircraft propulsion. Historically, aircraft design is based on extrapolation of past experience, meaning that a new aircraft is an improvement of existing designs. Since an all-electric aircraft would be based on new technologies, its design requires the creation of new tools. Physics-based models have to be developed for all the components of the aero-vehicle. This paper presents a physics-based model for superconducting motors to be used in a synthesis and optimization program commonly used by aircraft designers. The model is able to generate a motor design from propulsion requirements providing weight, volume, efficiency, armature temperature, etc. The temperature rise in the armature is an important limiting factor in superconducting motors; this is calculated through a thermal model of the armature based on equivalent lumped parameter circuit. All the electrical and thermal parameters are used for optimization of the motor. The paper presents a design example based on a typical small aircraft.

Notional all-electric ship thermal simulation and visualization

The impact of unpredicted or unplanned thermal disturbances on any future all-electric ship may well lead to unexpected and untimely failure of mechanical-electrical systems (e.g., power electronics, high power sensors, and pulsed weapons) to the detriment of the ships combat mission. The high power, rapid transients, and harsh environment expected to be imposed on both the electrical and thermal systems may well be unique to this class of ship. In order to develop the thermal analysis, a comprehensive visualization tool to display the temperature and heat dissipation distributions in the entire ship has been developed. This tool includes a simplified physical model, which combines principles of classical thermodynamics and heat transfer, resulting in a system of three-dimensional differential equations which are discretized in space using a three-dimensional cell centered finite volume scheme. Therefore, the combination of the proposed simplified physical model with the adopted finite volume scheme for the numerical discretization of the differential equations is called a volume element model (VEM). In this work, a 3D simulation is performed in order to determine the temperature distribution inside the ship for six different operating conditions. Visit visualization tool is used to plot the results.

Novel Integrated Energy Systems and Control Methods with Economic Analysis for Integrated Community Based Energy Systems

A framework for establishing neighborhood based integrated energy systems, which are highly but not solely dependant on renewable energy is presented. Integrated energy systems is a whole system concept. It includes knowledge, design, analysis, construction, and long term utilization of a communitys electrical, mechanical, thermal, educational, and governing systems at all levels, individual homes, communities, local and regional infrastructures, etc. The authors depend heavily on their experiences in the USA State of Florida. The guidance should be generally applicable to world wide communities with additional considerations. Integrated energy systems will provide significant benefit to both the community and the national grid infrastructure. These systems can provide economic, ideological, and aesthetic satisfaction to certain homeowners who believe a greater dependence on renewable energies is demanded by world events. These systems can also provide significant performance improvements for a national grid that faces rapidly increasing demand at a pace that surpasses the pace of additional transmission or even generation in some places of the world. In this paper we touch on considerations for the national infrastructure, the actual neighborhood microgrid design, the energy sources, the economic analysis and education. This is intended to be a start of discussion and not an exhaustive analysis or case for integrated energy systems.

System-level optimization of the sizes of organs for heat and fluid flow systems

Abstract In this paper we show that the sizes (weights) of heat and fluid flow systems that function on board vehicles such as aircraft can be derived from the maximization of overall (system level) performance. The total weight of the aircraft dictates its fuel requirement. The principle owes its existence to two effects that compete for fuel. Components, power plants and refrigeration plants operate less irreversibly when they are larger. Less irreversibility means less fuel needed for their operation. On the other hand, larger sizes add more to the mass of the aircraft and to the total fuel requirement. This tradeoff pinpoints optimal sizes. The principle is illustrated based on three examples: a power plant the size of which is represented by a heat exchanger, a counterflow heat exchanger without fluid flow irreversibility, and a counterflow heat exchanger with heat transfer and fluid flow irreversibilities. The size optimization principle is applicable to the organs of all flow systems, engineered (e.g., vehicles) and natural (e.g., animals).