Air-cooling used for metal cutting

American Journal of Applied Sciences, Feb, 2009 by Brian Boswell, Tilak T Chandratilleke

INTRODUCTION

In this research examines the operational effectiveness of a Ranque-Hilsch vortex tube being used to cool tool tip during machining. The Ranque-Hilsch vortex effect was discovered in the early 1930s when it caused considerable excitement, as it demonstrated that it was possible to produce hot and cold air by supplying compressed air to a tube. At first it is hard to believe that such a device can produce hot and cold air and at a useful flow rate. The vortex tube is a simple device with no moving parts, which simultaneously produces cold and hot air streams. However, to date, there is little research in determining the efficiency of using a vortex tube in cooling tool tips. Therefore, to establish the effectiveness of the heat transfer process on the tool tip a series of experimental investigations has been carried out. These tests will determined the most suitable parameters to use, like mass flow rate of cold and hot air, cold and hot tube diameter with respect to tube length, to achievable minimum cold air temperatures.

Air-cooling has never been taken seriously by the manufacturing industry due to the fact that for many years traditional cutting fluid has been shown to be effective in cooling tool tips during the machining processes. The outcome of this research will prove that air-cooling can replace traditional cutting fluid for many machining applications, without any reduction in tool life or reduction in quality of work piece surface finish. The introduction of using a Ranque-Hilsch Vortex Tube to provide cold air to the tool interface is shown to significantly improve the performance of air-cooling. Recorded tool tip interface temperatures clearly indicate that there is a highly significant reduction in tool tip temperature. This reduction in temperature slows the wear mechanisms as shown by the reduced flank wear when examined under a microscope. Therefore, monitoring the growth of the flank wear indicates the increased tool life when being air-cooled.

The Ranque-Hilsch vortex tube (1) is a remarkable device that is able to separate airflow into two different streams simultaneously, one hotter than the inlet air and the other cooler, without any moving parts being involved. The mechanism producing the temperature separation of cold air and hot air when passing through the vortex tube is not yet fully understood. This device has been described as Maxwell's demon, a fanciful means of separating heat from cold without work. The vortex tube basically consists of three pipes and a supply of compressed air to achieve a moderately low temperature at the cold outlet. Ranque (2) attempted to exploit the commercial potential for this strange device that produced hot and cold air with no moving parts. Unfortunately, this venture failed and the vortex tube slipped into obscurity.

The mechanism underlying the energy transfer from the cold to the hot flow remains elusive. However, there is debate even as to the basic physics of the phenomenon, while the majority of researchers suggest the mechanism is based on the interactions of turbulence, compressibility and shear work as shown by the analysis of Deissler and Perlmutter (3). Recent research has been divided into two categories. The first category termed as external studies were concerned with the performance of the tubes. It was found by Gulyaev (4) that the minimum ratio of the length of the tube to that of its diameter was thirteen. Other research suggested a ratio of forty to fifty for optimum operation. As for the diaphragm, the optimum dimension is a ratio of 2:3 for the diaphragm diameter to tube diameter.

The vortex tube consists of three important parts the mid-section where the air enters into the vortex generator (which increases the speed of the air), the cold tube and the hot tube as shown in Fig. 1. Normally the hot tube is about 350 mm long and at the end there is a conical valve which controls the amount of hot air escaping. On the right side of the vortex generator is the cold tube exit. Between the vortex generator and the cold tube there is a diaphragm, with a central hole that can be easily changed. Diaphragms with large or small holes can also increase or decrease the temperature obtained at the cold exit.

Considering the above vortex tube, the compressed air is supplied circumferentially into the tube at sonic speed and creates a cyclone (vortex) spinning at a million revolutions per minute. The air is forced to spin inward to the centre where it then escapes up along the hot tube as this path presents the least resistance to the airflow. The air continues to spin as it travels along the tube until it meets the conical valve where it turns part of the spinning air column (vortex) inside itself. The slower moving air inside column of the spinning air gives up its heat to the faster spinning outside column of air. The cold air travelling down the spinning air is now directed out the cold end of the vortex generator and the hot air is exhausted out of the other end of the vortex tube. Adjusting the conical valve built into the hot air exhaust can change the temperature of these two air streams to as low as -55[degrees]C as shown by Fig. 2.