Michel A. Saad

Effects of surface roughness on the average heat transfer of an impinging air jet

Abstract The effect of surface roughness of a uniformly heated plate on the average heat transfer characteristics of an impinging air jet was experimentally investigated. Two aluminum plates, one with a flat surface and the second with some roughness added to the surface were fabricated. The roughness took the shape of a circular array of protrusions of 0.5 mm base and 0.5mm height. A circular Kapton heater of the same diameter as the plates (70mm) supplied the necessary power. The surfaces of the plates were polished to reduce radiation heat losses and the back and sides insulated to reduce conduction heat losses. Temperatures were measured over a Reynolds number ranging from 9600 to 38500 based on flow rate through a 6.85mm diameter nozzle. The temperature measurements were repeated for nozzle exit-to-plate spacing, z/d, ranging from 1 to 10. The average Nusselt number for both cases was plotted versus the Reynolds number and their functional correlation was determined. The results indicate an increase of up to 6.0% of the average Nusselt number due to surface roughness. This modest increase provides evidence to encourage further investigation and characterization of the surface roughness as a parameter for enhancing heat transfer.

The new LM2500 Cheng cycle for power generation and cogeneration

Abstract The advanced general electric (GE) LM2500 Cheng cycle is a recent development to improve the performance of the LM2500 STIG cycle. The thermal efficiency increases by 7%, and the complexity of the plant is reduced appreciably due to the fully automated configuration. The system includes a self-regulated pressure HRSG, steam-air cooling of the turbine blades and a fully automated computerized process control to implement fast start-up and shut-down. The systems fast response allows full steam injection within 30 min. The operation is integrated by several layers of microprocessor architectural structure to allow the choice of the most economic operation that follows a fluctuating electric price of local utilities. This results in a reduction of the operating manpower and maintenance costs. The system capital cost is comparable to the simple cycle of the LM2500 GEN SET COGEN systems. Performance of the advanced Cheng cycle using the general electric LM2500 gas turbine indicates that the power output (about 27 MW) and efficiency (between 44 and 45%) remain fairly constant regardless of a high ambient temperature. Existing gas turbine plants can be retrofitted to operate on the Cheng cycle resulting in a true parallel steam and gas turbine combined cycle. Equipment diagrams showing typical one-unit arrangements for the 27.5 MW GE LM2500/CHENG are presented. In addition to power generation, the advanced Cheng cycle is particularly attractive for cogeneration due to its flexible use of excess steam in the combustion chamber.

Thermal Protection Studies of Plastic Films and Fibrous Materials

An experimental method of radiant heating was used to study the thermal protection properties of certain film and woven materials. Samples of these, placed in front of fiberglass batting containing a phenolic resin, were exposed to a radiant heat flux of 5 W/cm2 and the resin mass loss as a function of time indicated the thermal protective effectiveness of the film or woven material for the bare fiberglass insulation. The materials tested included aluminized and unaluminized synthetic plastic films and fibrous materials like silicon carbide and phenolic novolac. The yield strength of selected aluminized plastics at tem peratures ranging from room temperature to 260°C was also measured. Aluminized polyimide (KAPTON®) was shown to be an effective light weight heat shield exhibiting the lowest weight loss in the assembly and most favorable stress-strain properties. Polyetheretherketone (PEEK) and poly etherimide (ULTEM®) were somewhat less satisfactory than KAPTON®, mainly for reasons of mechanical failure at high temperatures. Aluminized polyester (MYLAR®) and polyvinylfluoride (TEDLAR®) were severely ther mally damaged by the radiant energy and the observed weight loss came from both the film and the phenolic resin contained in the fiberglass. Poly benzimidazole (PBI) film, while not thermally damaged, also lost weight when exposed to the same radiant heat source, but we attribute this to bound water released upon heating. For the fibrous materials, aluminized woven silicon car bide (NICALON®) was better than an aluminized phenolic novolac (KYNOL®) in reducing the resin weight loss of the fiberglass insulation.

Temperature distribution in a vaporizing droplet with internal heat generation

Abstract The time-dependent temperature distribution within a droplet with internal heat generation and surface vaporization has been studied using a finite-difference numerical method. The heat transfer equations were nondimensionalized for computer calculations, and solutions were obtained for cases when the initial temperature is equal to and different from the surface temperature. Variation of droplet radius due to vaporization was included in the assumed model, and radial temperature distributions, relative to the surface temperature, were calculated under the assumption of constant surface temperature. Representative results are given as temperature and radius histories for a wide range of dimensionless variables. The results are applicable when surface temperature changes with time provided that the rate of change is small compared to the initial temperature transient due to heat generation.