Dennis L. O’Neal

An analytical solution to transient heat conduction in a composite region with a cylindrical heat source

An approximate analytical solution to the transient heat-conduction problem in a large composite region with an internal cylindrical source is presented. The generalized orthogonal expansion technique is utilized in deriving the solution. Such problems are encountered in the design or simulation of the ground-coupled heat exchangers used in ground-coupled heat pumps. Solutions are presented for the nondimensional temperature as a function of the ratios of the thermal conductivities and thermal diffusivities of the two materials in the layers. To verify the correctness of the solution, comparisons are made between the two-layer composite solution and the classical homogeneous cylindrical source solution and a finite difference solution.

A Transient Heat and Mass Transfer Model of Residential Attics Used to Simulate Radiant Barrier Retrofits, Part I: Development

This paper describes a transient heat and mass transfer model of residential attics. The model is used to predict hourly ceiling heat gain/loss in residences with the purpose of estimating reductions in cooling and heating loads produced by radiant barriers. The model accounts for transient conduction, convection, and radiation and incorporates moisture and air transport across the attic. Environmental variables, such as solar loads on outer attic surfaces and sky temperatures, are also estimated. The model is driven by hourly weather data which include: outdoor dry bulb air temperature, horizontal solar and sky radiation, wind speed and direction, relative humidity (or dew point), and cloud cover data. The output of the model includes ceiling heat fluxes, inner and outer heat fluxes from all surfaces, inner and outer surface temperatures, and attic dry bulb air temperatures. The calculated fluxes have been compared to experimental data of side-by-side testing of attics retrofit with radiant barriers. The model predicts ceiling heat flows with an error of less than ten percent for most cases.

A Transient Heat and Mass Transfer Model of Residential Attics Used to Simulate Radiant Barrier Retrofits, Part II: Validation and Simulations

A computer program was developed and used to implement the model described on Part I of this paper. The program used an iterative process to predict temperatures and heat fluxes using linear algebra principles. The results from the program were compared to experimental data collected during a three-year period. The model simulated different conditions such as variations in attic ventilation, variations in attic ceiling insulation, and different radiant barrier orientations for summer and winter seasons. It was observed that the model predicted with an error of less than ten percent for most cases. This paper presents model results for nonradiant barrier cases as well as cases for radiant barriers installed horizontally on top of the attic floor (HRB) and for radiant barriers stapled to the attic rafters (TRB). Savings produced by radiant barriers and sensitivity analyses are also presented. The model results supported the experimental trend that emissivity was the single most significant parameter that affected the performance of radiant barriers.

Return Air Leakage Impact on Air Conditioner Performance in Humid Climates

An experimental study was conducted to quantify the effect of return air leakage from hot/humid attic spaces on the performance of a residential air conditioner. Tests were conducted in psychrometric facilities where temperatures and humidities could be controlled closely. The test air conditioner had a nominal cooling capacity of 12.3 kW and a seasonal coefficient of performance of 3.8. Return air leakage from hot attic spaces was simulated by assuming adiabatic mixing of the indoor air at normal conditions with the attic air at high temperatures. Effective capacity and coefficient of performance both decreased with increased return air leakage, leakage air temperature, and air humidity. Under attic conditions of 54.4°C and 20 percent relative humidity, 10% return leakage reduced the effective cooling capacity and coefficient of performance of the air conditioner by approximately 30%. Power consumption was relatively constant for all variables except outdoor temperature. The sensible heat ratio (SHR), which is a measure of the dehumidification performance, increased with increasing leakage.

Effect of Refrigerant Flow Control on the Heating Performance of a Variable-Speed Heat Pump Operating at Low Outdoor Temperature

An experimental study was conducted to investigate the effect of electronic flow control on the performance of a variable-speed heat pump. A heat pump with two different expansion devices (capillary tube and electronic expansion valve) was tested in a psychrometric calorimeter over a range of outdoor temperatures from -15 to 7°C. Heat pump performance was first optimized with respect to charge for each expansion device through cycle-matching tests. Parametric tests also were conducted by changing compressor speed and opening angle for the electronic expansion valve at each outdoor temperature. The refrigeration cycle characteristics of the electronic valve were illustrated using pressure-enthalpy diagrams. Performance enhancement was also analyzed in terms of superheat, heating capacity, and energy efficiency ratio (EER). Comparison of the capillary tube and electronic valve indicated that the superheat significantly improved when using the electronic valve. Also, unit showed larger heating capacity and EER with the electronic valve than with the capillary tube except when the compressor speed was above 95 Hz. Enhancement of heating performance became larger as outdoor temperature decreased.

Design for Reliability of Refrigerator Components Subject to Repetitive Loads in Use

A general method for reliability design of a mechanical system is proposed. A case study is presented for a frozen food drawer and handle system used in a residential refrigerator. The system had been failing when subjected to repetitive loads under normal consumer usage in the field. The mode and mechanism of the failure was fracturing which originated in a design flaw. Data from failed products in the field, accelerated life tests, and corrective action plans were used to identify the key control parameters for the drawer and handle system. The missing or improper design parameter in the original design was the system’s inability to endure the normal repetitive stresses in the field. After a tailored series of accelerated life tests with corrective action plans, the B1 life of the new drawer and handle system is now guaranteed to be over ten years with a yearly failure rate of 0.1 percent.Copyright

Reliability Design and Case Study of a Refrigerator Compressor Subjected to Repetitive Loads

A newly designed crankshaft of a compressor for a side-by-side (SBS) refrigerator was studied. Using mass and energy conservation balances, a variety of compressor loads typically found in a refrigeration cycle were analyzed. The laboratory failure modes and mechanisms were compressor locking and crankshaft wear. These were similar to those of the failed samples in the field. Failure analysis, accelerated life testing (ALT), and corrective actions were used to identify the key reliability parameters. The design parameters of the crankshaft included the hole locations and the groove of the crankshaft used for oil lubrication, crankshaft hardness, and thrust washer interference. Based on the analysis and design changes, the B1 life of the new design is now over ten years with a yearly failure

A Nondimensional Analysis of Vertical Configuration, Ground-Coupled Heat Pump Startup

A nominal 10.6 kW (three ton), vertical configuration, ground-coupled heat pump was installed in Abilene, TX in Dec. 1989 and has been monitored since then using a remote data acquisition system. Monitored data include: temperature and relative humidity of return and supply air, water temperature entering and leaving the condenser, power consumption of the individual system components, cycling rate, on-time, and soil temperatures at various depths and radial locations. Water and air flow rates have been measured twice during the monitored period, and have remained constant. The measured quantities allow calculation of instantaneous capacity, power, coefficient of performance (COP), and ground-coil heat rejection. Data for operation in the cooling mode are discussed herein. Based on the experimental data, it was discovered that the water temperature entering the condenser (EWT) exhibited a prolonged minimum after startup due to cooling of the water during the off-cycle. The decreased levels of EWT early in the cycle increased capacity and decreased power, both acting to increase the COP. Two sets of nondimensional groups were developed based on the experimental data. The first nondimensional group allowed analysis of startup independent of changes in independent variables, while the second nondimensional group quantified the performance benefits due to cyclic operation. The results tended to indicate that an improvement in COP may be achieved in cyclic operation, due to the decreased values of EWT early in the cycle. This is in contrast to behavior for air source heat pumps, where cycling always degrades efficiency.