Mark Ricklick

Sidewall Effects on Heat Transfer Coefficient in a Narrow Impingement Channel

This paper examines the local and averaged effects of channel height in the presence of sidewalls on heat transfer coefficients, with special attention given to the sidewall behavior. High resolution local heat transfer coefficient distributions on target and sidewall surfaces were computed using temperature sensitive paint, recorded via a scientific-grade charge-coupled device camera, and compared with available literature. This is important, as many of the major correlations related to impingement channels are directly applicable to wide arrays of jets, for which the influence of the presence of sidewalls is minimal. Streamwise pressure distributions were recorded and used to explain heat transfer trends and to determine the thermal effectiveness of each channel. Results are presented for average jet-based Reynolds numbers between 17,000 and 45,000. All experiments were carried out on a large-scale single-row 15-hole impingement channel (with an X/D of 5, a Y/D of 4, and a Z/D of 1, 3 and 5). It was observed that available correlations accurately predict the target surface heat transfer coefficients when the influence of the sidewall is minimal, typically at larger channel heights and lower Reynolds numbers. Smaller channel heights tend to outperform larger channels, when considering thermal effectiveness and channel-averaged heat transfer rates, due to the increased heat transfer on the channel sidewalls.

Solar Retrofit to Combined Cycle Power Plant With Thermal Energy Storage

Solar thermal power plants have been constructed over the past two decades to reduce harmful emissions and provide a long-term solution for oil independent electricity generation. Of the solar power plant solutions, Rankine cycle based machines have most widespread uses. This study focuses on the modeling of a solar retrofit to a typical combined cycle power plant. The goal is to operate the plant 17 hours per day, making use of thermal storage capability so that the plant may operate even during a portion of the night time. The plant will be located in Orlando, Florida to take advantage of the abundance of sun in that geographic location. On the cycle side, the amount of solar collectors, the working fluid, and the turbine are considered. The thermal storage system, on the other hand, must be designed based upon a balance between cost and storage density. A decision will be made from existing sensible heat solid storage materials. The storage material evens out the energy supplied to the turbine working fluid between the peak solar radiation of the day time and the absence of solar radiation at night. This plant can be implemented in two ways: as a completely newly constructed power plant or as an addition to a HRSG (Heat Recovery Steam Generation) configuration, which can be retrofitted to an existing combined cycle power plant to increase its overall efficiency. In this study, the addition of a solar air collection system with a storage unit to a HRSG combined cycle power plant is proposed. The HRSG will be designed using a series of energy balances for each component. This proposed plant will then be compared with a similar solar plant to examine its feasibility in terms of land area. The storage unit devised comprises 1377 m3 and stores approximately 3900 GJ of thermal energy, which equates to 8 hours of run time when solar radiation is not available. The benefit of this addition to the plant is that the storage reduces the gas turbine run time necessary to provide hot gas to the HRSG. The total cost of the storage medium is approximately

Effects of Channel Height and Bulk Temperature Considerations on Heat Transfer Coefficient of Wetted Surfaces in a Single Inline Row Impingement Channel

8 million.Copyright

Effect of Rib Aspect Ratio on Heat Transfer and Friction in Rectangular Channels

High performance turbine airfoils are typically cooled with a combination of internal cooling channels and impingement/film cooling. In such applications, the jets impinge against a target surface, and then exit along the channel formed by the jet plate, target plate, and side walls. Local convection coefficients are the result of both the jet impact, as well as the channel flow produced from the exiting jets. Numerous studies have explored the effects of jet array and channel configurations on both target and jet plate heat transfer coefficients. However, little work has been done in examining effects on the channel side walls, which may be a major contributor to heat transfer in real world applications. This paper examines the local and averaged effects of channel height and on heat transfer coefficients, with special attention given to the channel side walls. The effects on heat transfer results due to bulk temperature variations were also investigated. High resolution local heat transfer coefficient distributions on target and side wall surfaces were measured using temperature sensitive paint and recorded via a scientific grade charge-coupled device (CCD) camera. Streamwise pressure distributions for both the target and side walls was recorded and used to explain heat transfer trends. Results are presented for average jet based Reynolds numbers between 17,000 and 45,000. All experiments were carried out on a large scale single row, 15 hole impingement channel, with X/D of 5, Y/D of 4, and Z/D of 1, 3 and 5. The results obtained from this investigation will aid in the validation of predictive tools and development of physics-based models.Copyright

Comparison of Heat Transfer and Friction Augmentation for Symmetric and Non-Symmetric Wedge Turbulators on Two Opposite Walls

This paper is an investigation of the heat transfer augmentation in the fully-developed portion of a 2:1 aspect ratio channel with orthogonal ribs at Reynolds numbers based on the open channel hydraulic diameter of 20,000, 30,000, and 40,000. Ribs are applied to the two opposite wide walls. The rib aspect ratio is varied systematically from 1, 3, and 5, with a constant rib height and constant rib pitch (rib pitch-to-height ratio of 10). The purpose of the study is to extend the knowledge of the performance of rectangular channels with ribs to include high aspect ratio ribs. Data reported includes the local Nusselt number augmentation as well as the friction factor augmentation. With increasing rib width, the overall heat transfer augmentation decreased accompanied by a reduction in pressure drop.

Channel Height and Jet Spacing Effect on Heat Transfer and Uniformity Coefficient on an Inline Row Impingement Channel

This paper is an investigation of the heat transfer and friction augmentation in the fullydeveloped portion of a narrow rectangular duct (AR=2) with wedge turbulators applied to the top and bottom walls. Tests are conducted at 10,000, 20,000, 30,000, and 40,000 Reynolds numbers based on the channel hydraulic diameter. The purpose of the paper is to find the overall thermal performance of four different wedge-shaped transport promoters, two symmetric and two non-symmetric wedges varying in height and footprint. Experimental setup consists of 40 segmented and individually heated copper blocks (10 for each wall) with temperature data measured with thermocouples embedded in each block on all four walls. Data reported includes the Nusselt number augmentation of the side walls of the channel in addition to the top and bottom featured walls to quantify the influence of flow disturbances caused by these wedge geometries to the surrounding smooth walls. Overall thermal performance is presented for each case to determine which wedge shape contributes to high heat transfer with a lower pressure loss. The non-symmetric (half) wedge shapes resulted in heat transfer augmentations 30% to 50% lower than the symmetric (full) wedge cases, and 20% to 50% lower friction factor augmentations. A better understanding of the effects produced by these geometries will help in the design and development of more effective cooling-channel design.

Comparison of Heat Transfer Prediction for Various Turbulence Models in a Pin Fin Channel

Future high performance turbine airfoils will likely be cooled in a near wall configuration, potentially employing a combination of narrow, distributed internal cooling channels and impingement. In such applications, the jets impinge against a target surface, and then exit along the channel formed by the jet plate, target plate, and side walls. Local convection coefficients are the result of both the jet impact, as well as the channel flow produced from the exiting jets and the complex interaction between the jet and the cross flow. Numerous studies have explored the effects of jet array and channel configurations on both target and jet plate heat transfer coefficients, yet with little consideration of thermal stress related effects. A detailed study on the uniformity coefficient that these jets and cross flow generate on the surface is carried out. It is important to maintain a high uniformity coefficient while still having a high heat transfer coefficients to reduce thermal stresses. It is also important to use as little flow as possible while maintaining a high heat transfer coefficient. The study presented experimentally investigates the effects of wall height, jet Reynolds number, and jet spacing on the Nusselt number and uniformity of a narrow inline row impingement channel. The channel height was set at 1, 3, and 5 diameters, jet spacing was 5 and 15 diameters, and the channel width was kept constant at 4 diameters. Although heat transfer coefficients are highly sensitive to the jet Reynolds number and channel height, the uniformity of the distribution is mainly governed by the channel height and jet spacing. A channel height of 3 jet diameters tended to produce the best uniformity coefficients, regardless of the jet to jet spacing; with side walls out performing target surfaces.Copyright

A Study of Heat Transfer Augmentation for Recuperative Heat Exchangers: Comparison Between Three Dimple Geometries

Heat transfer augmentation through the use of pin fins is commonly employed in gas turbine blades and vanes. This is particularly true in the trailing edge region, where the pin fins provide additional structural support, flow control, and heat transfer enhancement. In the industrial design process, commercial CAE packages are now routinely used for predicting the performance of such cooling configurations. The designer is often left with the timeconsuming task of validating their approach. The present study will investigate the accuracy of a selection of the turbulence models commercially available, comparing Nusselt numbers against those found in selected, well-established articles from the literature. With the industrial importance of simulation turn-around time in mind, this investigation will compare 4 RANS turbulence models, at various levels of mesh refinement and temporal resolution. Turbulence models investigated are: realizable k-e, Menter’s k-ω SST, the v 2 -f model, and a quadratic formulation of the realizable k-e model. The geometry investigated is a staggered, 10 row pin-fin channel, with S/D= 2.19, H/D= 0.875, X/D=1.32, at a Reynolds number of 52.8k. Results show that quadratic versions of the realizable k-e model tend to match between 6% and 21% of the experimental Nusselt number results on the surface of the pin.

The Effect of Transpiration on Discrete Injection for Film Cooling

This study presents an investigation of the heat transfer augmentation for the purpose of obtaining high effectiveness recuperative heat exchangers for waste heat recovery. The focus of the present work is in the fully developed portion of a 2:1 aspect ratio rectangular channel characterized by dimples applied to one wall at channel Reynolds numbers of 10,000, 18,000, 28,000, and 36,000. The dimples are applied in a.staggered-row, racetrack configuration. In this study, a segmented copper test section was embedded with insulated dimples in order to isolate the heat transfer within the dimpled feature. The insulated material used to create a dimpled geometry isolates the heat transfer within the dimple cavity from the heat transfer augmentation on the surrounding smooth walls promoted by the flow disturbances induced by the dimple. Results for three different geometries are presented, a small dimple feature, a large dimple, and a double dimple. The results of this study indicate that there is significant heat transfer augmentation even on the nonfeatured portion of the channel wall resulting from the secondary flows created by the features. Overall heat transfer augmentations for the small dimples are between 13―27%, large dimples between 33―54%, and double dimples between 22―39%, with the highest heat transfer augmentation at the lowest Reynolds number for all three dimple geometries tested. Heat transfer within the dimple was shown to be less than that of the surrounding flat regions at low Reynolds numbers. Results for each dimple geometry show that dimples are capable of promoting heat transfer over the entire bottom wall surface as well as the side walls; thus the effects are not confined to within the dimple cavity.

Internal Cooling Using Porous Turbulators: Heat Transfer and Pressure Drop Measurements

A segment of permeable wall is installed near a row of cylindrical film holes, parallel to the flow and inclined at 35 degrees. Coolant is forced through both the permeable wall and the film holes resulting in a downstream film composed of both transpired and discretely injected coolant. The permeable wall extends 1.5 cylindrical hole diameters in the flow direction. The effects on the aerodynamic performance and cooling downstream of the row of cylindrical holes in the presence of transpiration is studied numerically with a procedure validated by hot-wire anemometer and temperature sensitive paint measurements. The hydrodynamic boundary layer in the presence of film and adiabatic film cooling effectiveness downstream of single and coupled film sources are compared with numerical predictions. The performance of the coolant film is predicted in order to understand the sensitivity of cooling and aerodynamic losses on the relative positioning of the two sources at each blowing ratio. The results indicate that a coupling of the two sources allows a more efficient use of coolant by generating a more uniform initial film. With careful optimization the discrete holes can be placed farther apart laterally and operate at a lower blowing ratio with a transpiration segment making the large deficits in cooling effectiveness mid-pitch less severe, overall minimizing coolant usage. Comparisons of linear superposition predictions of the two independent sources with the corresponding coupled scenario indicate the two films positively influence one another and surpass additive predictions of cooling. All relative placements have an overall beneficial effect on the cooling seen by the protected wall. Some cases show an increase in area-averaged film cooling effectiveness of 300% along with a 50% increase in aerodynamic loss coefficient by injecting an additional 10% coolant. In this study the downstream transpiration placement is found to perform best of the three geometries tested while considering cooling, aerodynamic losses, local uniformity and manufacturing feasibility. With further study and optimization this technique can potentially provide more effective thermal protection at a lower cost of aerodynamic losses and spent coolant.Copyright