David A. Yashar

An Investigation of Refrigerant Void Fraction in Horizontal, Microfin Tubes

A series of experiments to determine void fraction in both condensation and evaporation have been performed in horizontal microfin tubes with refrigerants R134a and R410A. Mass flux varied from 75 kg/m2·s to 700 kg/m2·s and average test section quality varied from 5% to 80%. Zero degree (axial grooving) and 18 degree helix microfin tube configurations have been examined in both condensation and evaporation. Four tubes were examined (7.3 mm and 8.9 mm diameters with axial and 18 degree helix angle microfins). Evaporation and adiabatic conditions in microfin tubes generally show similar void fraction trends found in smooth tubes. Condensation results for microfin tubes show a lower void fraction than smooth tubes.

An Optimized Design of Finned-Tube Evaporators Using the Learnable Evolution Model

Optimizing the refrigerant circuitry for a finned-tube evaporator is a daunting task for traditional exhaustive search techniques due to the extremely large number of circuitry possibilities. For this reason, more intelligent search techniques are needed. This paper presents and evaluates a novel optimization system called ISHED1 (intelligent system for heat exchanger design). This system uses a recently developed non-Darwinian evolutionary computation method to seek evaporator circuit designs that maximize the capacity of the evaporator under given technical and environmental constraints. Circuitries were developed for an evaporator with three depth rows of 12 tubes each, based on optimizing the performance with uniform and nonuniform airflow profiles. ISHED1 demonstrated the capability to generate designs with capacity equal or superior to that of best human designs, particularly in cases with non-uniform airflow.

An experimental and computational study of approach air distribution for a finned-tube heat exchanger

The distribution of air flow approaching a finned-tube heat exchanger is one of the predominant factors influencing the heat exchangers performance. This article describes a method for measuring and predicting the inlet air flow distribution using particle image velocimetry (PIV) and a computational fluid dynamics (CFD) model, highlighting the source and magnitude of air side maldistribution. The studied case was a single-slab, four-depth-row, louvered-fin heat exchanger installed vertically in a horizontal duct. The measured data showed that the air approaching this very simple test case generally maintained velocities of 1.25 ms−1 (4.1 fts−1) to 1.35 ms−1 (4.4 fts−1), but certain portions of the coil were completely obstructed, resulting in no air flow, and other portions realized velocities of over 1.7 ms−1 (5.6 fts−1). A CFD model of the air flow through this heat exchanger was developed based on a momentum resistance modeling approach. The CFD results agreed well with the PIV measurements, predicting the local velocities within 3% over most of the domain and within 10% in areas with the largest velocity gradients.

A dual-mode evolutionary algorithm for designing optimized refrigerant circuitries for finned-tube heat exchangers

Heat exchanger performance is strongly influenced by the refrigerant circuitry, i.e., the connection sequence of the tubes. This article describes an evolutionary computation-based approach for designing an optimized refrigerant circuitry used in an intelligent system for heat exchanger design. The technique used in this design employs two methods to generate designs implemented separately in two modules: the knowledge-based evolutionary computation module and the symbolic-learning-based evolutionary computation module. The optimization example presented in this article employed each module independently and used the combined approach to demonstrate the performance of each module and the power of the combined module approach. The best circuitry designs determined through these optimization runs yielded substantial capacity improvements over the original design; the symbolic-learning- and knowledge-based modules returned circuitry designs that improved the heat exchanger capacity by 2.6% and 4.8%, respectively, while the combined module approach resulted in a circuitry design that improved the capacity by 6.5%.

An experimental and computational study of approach air distribution for slanted and A-shaped finned-tube heat exchangers

One of the most influential factors of the performance of a finned-tube heat exchanger is the distribution of the air passing through it; therefore, it must be known in order to produce a highly efficient design. We examined two different common style air-to-refrigerant, finned-tube heat exchangers: a single-slab coil oriented at an angle of 65° to the duct wall and an A-shaped coil with an apex angle of 34°. We used particle image velocimetry (PIV) to measure their in-situ airflow distributions. The results show that the airflow distributions for both heat exchangers are highly nonuniform with different sections being subject to vastly different air velocities. We also used a momentum resistance-based computational fluid dynamics (CFD) approach to model the airflow distributions through these heat exchangers. The modeled results agreed with the measured values, with most of the simulated velocities falling within +/-10% of the measured velocities. The results of this study show that the velocity profile for any configuration is strongly influenced by the geometry of the heat exchanger and other features in its proximity and, therefore, each installation configuration will have its own unique velocity distribution. The information presented in this paper documents the maldistribution of airflowing through finned-tube heat exchangers and highlights the sources and magnitude of the nonuniformities.

A Microfabricated Flow Controller for Refrigerant Expansion

A flow controller for refrigerant expansion is reported. Devices are fabricated using a micromolding technique that is developed for thick nickel electrodeposition. The device consists of a short-tube restrictor with an integrated normally open valve, which, when actuated, presents a controllable blockage into the flow passage to obstruct the flow. Fabricated devices are evaluated with compressed air, with up to 22% reduction in mass flow rate at maximum actuation of the restrictor. The devices are also evaluated in an R134a vapor compression system of 1.5-2 kW, with the ability to control mass flow that is found to be greatly influenced by the vapor compression cycles operational parameters. After the inlet pressure, the level of subcooling proved to be the most important parameter. For a cycle operating between 29 deg C and 4 deg C , saturation temperatures in the condenser and evaporator, respectively, actuation of the device reduced the refrigerant mass flow rate by 3.5% with 0.6 oC subcooling and up to 10.8 % with 5 deg C subcooling.