M.V. Krishna Murthy

An improved maximum power point tracker using a step-up converter with current locked loop

It is well known that for a given solar radiation intensity and solar cell temperature there exists a Maximum Power Point at which the power generated from the PV panel is at its maximum. A system designer is interested in optimal matching of the load to the PV generator so that the maximum power can be obtained during operating period. A Maximum Power Point Tracker (MPPT) using a step up converter with a current locked loop is developed. Its performance is compared with the literature (the step down power converter using PWM technique), under different solar irradiance and ambient temperatures. It showed an improvement in the output power by 22.5% (average) over a wide range of solar irradiation in a day.

Instrumentation to measure thermal contact resistance

Instrumentation was developed to measure the thermal contact resistance across metallic contacts by varying the contact pressure in different environments. Experiments were conducted on oxygen free, high conductivity (OFHC) copper cylindrical flat contacts and the measured thermal contact resistance is in good agreement with the theoretical models reported in the literature. The thermal conductivity of OFHC copper and stainless steel was measured at different temperatures and these values are in good agreement with the literature values.

Forced-air precooling of spherical foods in bulk: A parametric study

In this paper, a mathematical model for forced-air precooling of spherical food products in bulk is developed. The foods are arranged in horizontal layers stacked one above the other to form a rectangular parallelepiped with a vertical gap in between the product layers. The foods are cooled by chilled air blown along the height of the package. The governing equations for the conduction heat transfer in the foods, simultaneous heat and mass transfer at the food-air interface and in the air stream are solved numerically using finite-difference methods. A comprehensive numerical study is performed by varying the process parameters over a wide range. Typical results showing the variation of moist air properties along the height of the package and the effect of each parameter on the process time are presented. The ranges of parameters for advantageous operation of the precooling system are identified. Correlations are obtained for the process time based on the product center and mass-averaged temperatures in terms of process parameters.

Combined natural convection heat and mass transfer from vertical fin arrays

Natural convection transport processes play an important role in many applications like ice-storage air-conditioning. A mathematical formulation of natural convection heat and mass transfer over a shrouded vertical fin array is developed. The base plate is maintained at a temperature below the dew point of the surrounding moist air. Hence there occurs condensation of moisture on the base plate, while the fins may be partially or fully wet. A numerical study is performed by varying the parameters of the problem. The local and average Nusselt numbers decrease in streamwise direction and tend to approach fully developed values for sufficiently large values of the fin length. The results show that beyond a certain streamwise distance, further fin length does not improve the sensible and latent heat transfer performance, and that if dry fin analysis is used under moisture condensation conditions, the overall heat transfer will be underestimated by about 50% even at low buoyancy ratios.

Absorption-resorption heating cycles with the new working pairs R21-NMP and R21-DMA

The new working pairs, R21-NMP and R21-DMA, find potential application as working pairs for the single stage Resorption Heat Pump (RHP) and the Resorption Heat Transformer (RHT) cycles. A thermodynamic cycle analysis with these pairs shows that single stage RHPs have high COPs in their entire range of operation. RHTs show higher temperature boosts (up to 47 K) than the simple absorption heat transformers. Absorber temperatures of up to 400 K can be achieved in a single stage RHT system using R21 as the refrigerant. However, absorption-resorption systems have inherent limitations on the range of operating temperatures. Besides, they necessitate a higher pump work as compared to simple single stage absorption heating systems.

Thermodynamic modelling of absorption-resorption heating cycles with some new working pairs

This paper presents the results of a thermodynamic cycle analysis of single stage resorption heat pump (RHP) and resorption heat transformer (RHT) cycles with the new working pairs R22-NMP and R22-DMA. The coefficients of performance (COP) are correlated with the low grade source temperature, temperature at which useful heat is obtained and ambient temperature. The COPs are in the range 1.20–1.60 for the RHP mode and 0.25–0.45 for the RHT mode. Absorber temperatures (useful temperatures) as high as 50°C in the RHP mode and 87°C in the RHT mode have been obtained. It is observed that absorption-resorption systems are inflexible in their range of operating temperature and necessitate a higher pump work as compared with simple single-stage absorption heating systems. However, single stage RHTs show higher temperature boosts than simple absorption heat transformers.

Analysis of heat and mass transfer during bulk hydraircooling of spherical food products

Hydraircooling is a technique used for precooling food products. In this technique chilled water is sprayed over the food products while cold unsaturated air is blown over them. Hydraircooling combines the advantages of both air- and hydrocooling. The present study is concerned with the analysis of bulk hydraircooling as it occurs in a package filled with several layers of spherical food products with chilled water sprayed from the top and cold unsaturated air blown from the bottom. A mathematical model is developed to describe the hydrodynamics and simultaneous heat and mass transfer occurring inside the package. The non-dimensional governing equations are solved using the finite difference numerical methods. The results are presented in the form of time-temperature charts. A correlation is obtained to calculate the process time in terms of the process parameters.

Thermal Conductivity and Thermal Contact Conductance Studies on Al12%Si10wt.%SiCp Metal Matrix Composites

Al12%Si10wt.%SiCp is a metal matrix composite (MMC) which is replacing the traditional metals in automobile industry. This paper describes the detailed investigations carried out for the first time on thermal contact resistance across this MMC contacts and also across the contacts between this MMC and other engineering materials like stainless steel, aluminum and copper, in vacuum at different contact pressures. The experimental results are compared with the theoretical models available in the literature for metal contacts. It is found that MMC contacts also show a behavior of metallic contacts in respect of thermal contact conductance i.e., the contact conductance increases as a function of contact pressure and number of load cycles. The thermal conductivity of the MMC is measured at different temperatures and is found to be invariant with temperature up to 100 C.

Time-Temperature Histories of Spherical Food Products during Bulk Forced-Air Precooling

A general mathematical model for forced air precooling of spherical food products in bulk is developed. The food products are arranged inline to form a rectangular parallelepiped. Chilled air is blown along the height of the package. The governing equations for the transient two-dimensional conduction with internal heat generation in the product, simultaneous heat and mass transfer at the product-air interface and one-dimensional transient energy and species conservation equations for the moist air are solved numerically using finite difference methods. Results are presented in the form of time-temperature histories. Experiments are conducted with model foods in a laboratory scale air precooling tunnel. The agreement between the theoretical and experimental results is found to be good. In general, a single product analysis fails to predict the precooling characteristics of bulk loads of food products. In the range of values investigated, the respiration heat is found to have a negligible effect.

A Markov chain model for a turbomolecular pump: theory and experiments

The random flow of molecules in turbomolecular pump is proved to be of Markovian type stochastic process. Two new variables-mean fast passage time and long range equilibrium probability-are related to the maximum compression ratio and the maximum flow, theoretically. An enumeration method to identify the flow pattern is adopted. The overall effective compression ratio is proved to be equal to the partial pressure ratio due to the molecules taking any one of the first passage time flow pattern for either direction of flow. A unified concept to make a selection of flow events is suggested to be included in the calculation of the probability and for the comparison of results. The influence of the backing pump and the flow at the ultimate vacuum level are quantified for the design purpose and are experimentally verified.