Expanded microchannel heat exchanger: design, fabrication and preliminary experimental test

This paper first reviews non-traditional heat exchanger geometry, laser welding, practical issues with microchannel heat exchangers, and high effectiveness heat exchangers. Existing microchannel heat exchangers have low material costs, but high manufacturing costs. This paper presents a new expanded microchannel heat exchanger design and accompanying continuous manufacturing technique for potential low-cost production. Polymer heat exchangers have the potential for high effectiveness. The paper discusses one possible joining method - a new type of laser welding named "forward conduction welding," used to fabricate the prototype. The expanded heat exchanger has the potential to have counter-flow, cross-flow, or parallel-flow configurations, be used for all types of fluids, and be made of polymers, metals, or polymer-ceramic precursors. The cost and ineffectiveness reduction may be an order of magnitude or more, saving a large fraction of primary energy. The measured effectiveness of the prototype with 28 micron thick black low density polyethylene walls and counterflow, water-to-water heat transfer in 2 mm channels was 72%, but multiple low-cost stages could realize the potential of higher effectiveness

Ventilation Air Preconditioning Systems

Increased outside ventilation air requirements demand special attention to how that air will be conditioned. In winter, the incoming air may need preheating; in summer. the mixed air may be too humid for effective dehumidification. Part-load conditions pose greater challenges: systems that cycle on and off allow unconditioned air into the building during compressor off-cycles. The Electric Power Research Institute has teamed with manufacturers to develop dual path HVAC systems, with one path dedicated to preconditioning the outside air. This paper discusses two such systems for cooling and dehumidification applications: one with a separate preconditioning unit and one with separate ventilation and return air paths in a single unit. Both deep-cool and deep-dry the incoming air before mixing it with the return air, thereby eliminating the latent load on the primary cooling coil. As unitary packages, they are easy to install in new and retrofit applications. Also, their excellent energy efficiency cuts electric energy consumption, providing significant operating savings

A thermal-optical analysis of a compound parabolic concentrator for single and multiphase flows, including superheat

A thermal and optical analysis of the performance of a refrigerant charged Compound Parabolic Concentrator (CPC) for solar applications operating in non-boiling, boiling and super-heated regimes is presented. The performance of the CPC working under these single and multiphase conditions is governed by the axial fractional channel lengths of the non-boiling and the superheating regions. The overall thermal loss coefficient, the dimensionless capacitance rate and collector efficiency factors for various CPC operating regions are defined. A new “Generalized Heat Removal Factor“, ℱ s for solar collectors under any operation mode is developed. The thermal efficiency of a CPC and flat-plate collector, whether under non-boiling, boiling or superheated conditions, is evaluated using ℱ s which enables the selection of a suitable collector design and concentration ratio at some specified operational temperature. It is shown that, in general, a CPC has a greater thermal conversion efficiency than a flat-plate for a given operating condition. Es wird eine thermische und optische Analyse des Verhaltens eines Verbund-Parabol-Kollektors für die Anwendung der Sonnenenergie vorgestellt, der mit Kältemittel im nichtsiedenden, und übehitzten Bereich arbeitet. Das Verhalten dieses unter ein- und mehrphasigen Bedingungen arbeitenden Kollektors wird bestimmt durch den axialen Anteil der Kühl-kanallängen im nichtsiedenden und im überhitzten Zustand. Es werden der mittlere thermische Verlustkoeffizient, die dimensionslose Wärmekapazität sowie die Kollektorwirkungsgrade für verschiedene Zustandsbereiche dieses Parabolspiegels definiert. Ein neuer „verallgemeinerter Wärmeabflußfaktor“, ℱ s , für Sonnenkollektoren, die unter beliebigen Betriebsbedingungen arbeiten, wurde entwickelt. Mit diesem ℱ s Faktor werden der thermische Wirkungsgrad des Parabolkollektors und eines Platten-kollektors bei einphasiger flüssiger Strömung beim Sieden und für überhitzten Dampf berechnet, wodurch es möglich wird, eine geeignete Kollektorauslegung und das dazugehörige Konzentrationsverhältnis bei vorgegebenen Betriebstemperaturen zu wählen. Es wird gezeigt, daß im allgemeinen der parabolische Kollektor einen höheren thermischen Wirkungsgrad besitzt als der Platten-kollektor bei identischen Betriebsbedingungen.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/46656/1/231_2005_Article_BF01377577.pd

Expanded microchannel heat exchanger: Design, fabrication, and preliminary experimental test

This article first reviews non-traditional heat exchanger geometry, laser welding, practical issues with microchannel heat exchangers, and high effectiveness heat exchangers. Existing microchannel heat exchangers have low material costs, but high manufacturing costs. This article presents a new expanded microchannel heat exchanger design and accompanying continuous manufacturing technique for potential low-cost production. Polymer heat exchangers have the potential for high effectiveness. This article discusses one possible joining method - a new type of laser welding named \u27forward conduction welding\u27, used to fabricate the prototype. The expanded heat exchanger has the potential to have counter-flow, cross-flow, or parallel-flow configurations, be used for all types of fluids, and be made of polymers, metals, or polymer-ceramic precursors. The cost and ineffectiveness reduction may be an order of magnitude or more, saving a large fraction of primary energy. The measured effectiveness of the prototype with 28 μm thick black low-density polyethylene walls and counterflow, water-to-water heat transfer in 2 mm channels was 72 per cent, but multiple low-cost stages could realize the potential of higher effectiveness. © IMechE 2012

Simplified Air Change Effectiveness Modeling

This paper describes recent progress in developing practical air change effectiveness modeling techniques for the design and analysis of air diffusion in occupied rooms. The ultimate goal of this continuing work is to develop a simple and reliable method for determining heating, ventilating, and air-conditioning (HVAC) system compliance with ventilation standards. In the current work, simplified two-region models of rooms are used with six occupancy patterns to find the air change effectiveness. A new measure, the apparent ACH effectiveness, yields the relative ventilation performance of an air diffusion system. This measure can be used for the prediction or evaluation of outside air delivery to the occupants. The required outside air can be greater or less than that specified by ventilation standards such as ASHRAE Standard 62-89

Finite Difference Heat Exchanger Model: Flow Maldistribution with Thermal Coupling

Channels not receiving the same amount of flow (flow maldistribution) is an important effectiveness loss for high effectiveness heat exchangers. This paper develops a finite difference model of a counter flow heat exchanger. It reproduced the simple NTU result for two channels exactly. For more than two channels, there is an edge effect, and the model agreed within 0.1% of the literature. Perfect agreement was achieved for a simple flow maldistribution case. For independent pairs of hot and cold channels, as flow rate is reduced, the effectiveness asymptotically approaches the irregularity parameter (a measure of channel flow maldistribution). With thermal coupling between pairs of channels (measured by an equivalent Peclet number), effectiveness continues to increase with decreasing flow rate. These results hold for uncorrelated flow maldistribution on both sides or no flow maldistribution on one side. However, when flow maldistribution is positively correlated, effectiveness is higher; and, when it is negatively correlated, effectiveness is lower. A number of resulting graphs illustrate the effectiveness for each channel with different flow rate and correlation. While one exemplary application is a polymer expanded microchannel heat exchanger, the model could be used for other heat exchangers

Parabolic Trough Receiver Heat Loss Testing (Poster)

Parabolic trough receivers, or heat collection elements (HCEs), absorb sunlight focused by the mirrors and transfer that thermal energy to a fluid flowing within them. Thje absorbing tube of these receivers typically operates around 400 C (752 F). HCE manufacturers prevent thermal loss from the absorbing tube to the environment by using sputtered selective Cermet coatings on the absorber and by surrounding the absorber with a glass-enclosed evacuated annulus. This work quantifies the heat loss of the Solel UVAC2 and Schott PTR70 HCEs. At 400 C, the HCEs perform similarly, losing about 400 W/m of HCE length. To put this in perspective, the incident beam radiation on a 5 m mirror aperture is about 4500 W/m, with about 75% of that energy ({approx} 3400 W/m) reaching the absorber surface. Of the 3400 W/m on the absorber, about 3000 W/m is absorbed into the working fluid while 400 W/m is lost to the environment