GB2582600A

GB2582600A - Method to improve the performance of gas measurement devices at low sample pressure

Info

Publication number: GB2582600A
Application number: GB1904237.3A
Authority: GB
Inventors: Dinsdale Steven; Hands Timothy; Daniel Payne Simon
Original assignee: Cambustion Ltd
Current assignee: Cambustion Ltd
Priority date: 2019-03-27
Filing date: 2019-03-27
Publication date: 2020-09-30
Anticipated expiration: 2039-03-27
Also published as: GB2582600B; GB201904237D0

Abstract

A sample probe 2 takes a sample from a source 1 such as an engine. The sample probe supplies sample gas to a gas analyser 9 via a thin capillary tube 8, prior to which the sample probe expands in diameter at 3 to reduce the velocity of flow past the entrance of said capillary 4. Between the expansion 3 and the entrance 4 is an obstruction or turbulator 10. Downstream of capillary tube 8 is a chamber 6 held at constant pressure connected to a vacuum source 7, to keep the pressure at 4 and subsequent flow into the capillary 8 constant. A flame ionisation or chemiluminescence detector may be included. Infrared lights and detectors may be included to determine concentration of a gas species. A diffuser 15 may also be included.

Description

Method to improve the performance of gas measurement devices at low sample pressure This invention relates to fast response gas analysers, where the measurement needs to be unaffected by the source pressure over as wide a range of pressure as possible. It especially relates to circumstances in which sample needs to be obtained from sub-atmospheric conditions, such as from the intake to an engine. One example of where it is desirable to measure gas concentrations at the engine intake is when studying exhaust gas recirculation (EGR) systems.

In a prior invention [US5073753] a fast response Flame Ionisation Detector (FID) for measuring the concentration of hydrocarbons (11C) was described. Figure 1 a and particularly insert 4 from that patent is reproduced schematically as Figure 1. A sample probe (2) extracts exhaust gas from point (1), usually from an engine, either directly from the cylinder or from the exhaust manifold. The downstream end of the sample probe (1) is connected to a chamber held at constant pressure (6), connected to a vacuum source (7). Close to the constant pressure chamber (6), a thin capillary tube (8) draws off gas from the sample probe, to supply a FID analyser (9). The narrowness of said capillary (8) keeps the frequency response of the instrument high by reducing mixing. Tube element (5) needs to extend briefly beyond the end (4) of the capillary (8) to avoid gas from the constant pressure chamber (6) being sampled, but element (5) is kept very short such that the pressure at (4) is essentially the same as in the constant pressure chamber (6). Keeping the pressure constant at (4) keeps the flow in the capillary (8) constant which keeps the response of the FID analyser (9) independent of sample pressure at (1).

In order to maintain a high frequency response, the sample probe (2) is usually a thin capillary tube to mitigate sample mixing along the probe length. However, the speed of gas in such a capillary will be high, and to reduce sample flow dependent changes in pressure at point (4) prior to the capillary (8) leading to the FID (9), it is necessary to expand the diameter of the sample probe at (3) to reduce the flow velocity.

Under conditions of low sample pressure (but not lower than the constant pressure chamber (6) at which point no sample would flow), significant deviations from the desired behaviour (i.e. that the flow in tube (8) is independent of the sample pressure at (1)) have been observed in real-world applications of gas analysers. An example is shown in Figure 2. It has been established using gas tracing that this phenomenon is not due to sampling of gas from chamber (6) due to momentary reverse flow. The reason for the behaviour is complex, but it must be due to the flow exiting from tube (2) failing to form, in a short enough distance, a uniform flow in the tube element from (3) to (5).

Examination of the literature concerning such flows [e.g. Khodaparast, Borhani and Thome, DOI 10.1007/s10404-013-1321-7], at the relatively low Reynolds numbers pertaining to these low sample pressures, indicates that a jet from the sample line (2) can persist for many tube diameters. This jet (illustrated in Figure 3) may attach to the wall of the tube from (3) to (5), its high velocity causing a drop in pressure at the entrance (5) to capillary tube (8) leading to reduced flow in said capillary and the diminution in signal level from analyser (9), as exampled in Figure 2. The same literature also indicate that the jet would quickly expand to fill the tube from (3) to (5) when transitional Reynolds numbers are reached. Thus encouraging turbulent flow would force the jet to immediately expand to fill the tube.

This patent relates to a method of avoiding the problem described above. Figure 4 shows one embodiment of the invention. A partial obstruction (10) has been introduced into the flow exiting from tube (2). The effect of this is to produce strong vorticial structures immediately behind the obstruction, which cause the flow to rapidly fill tube section (3) to (5), and lead, rapidly to well-developed flow (though perhaps not fully developed). The obstruction will be dubbed a "turbulator". This term is generally defined as a device which turns a laminar flow into a turbulent flow. The turbulator produces a partial flow blockage leading to rapid and reliable eradication of any jet of flow exiting from tube (2), and thus rapid and reliable development of a well-developed flow in tube (3) through to (5). Though it might be said that the function of the turbulator is to rapidly and reliably produce "uniform" flow in tube (3) to (5), of course the flow can never become truly uniform, in the sense that there is always a velocity profile due to wall shear. This then improves the pressure isolation at point (4), ameliorating the undesirable effect discussed above at low sample pressure.

Figure 5 shows a practical embodiment of such a turbulator, consisting of a partial obstruction in the form of a bar (2.0 x 0.2 x 0.1 mm) mounted in a much larger orifice plate. The orifice plate is essentially a way of holding the bar, and allowing it to be easily removed for cleaning if engine soot were to accrete on it and also to allow cleaning of the rest of the sampling system downstream of the turbulator. The notches C allow consistent re-orientation of the turbulator after removal.

Figure 6 shows that the pressure deviation shown in Figure 2 at low pressure is corrected, when using the turbulator illustrated in Figure 5.

There may be a plurality of gas analysers connected to a single sampling system. For example, Figure 7 shows an embodiment where two capillary tubes (8) are connected to the sampling system (2,3,4,5,6,7) after the turbulator (10), each supplying pressure isolated sample flow to a separate gas analyser (9). The gas analysers (9) are not limited to being FIDs for hydrocarbon detection. Any gas analyser which is sensitive to the rate of sample delivery would benefit from this pressure isolation method, for example Chemiluminescence Detectors (CLDs) for nitrogen oxide.

Figure 8 shows, in cross section, an alternative embodiment where a further gas sensor device is added downstream, i.e. in series; in this example an optical absorption type. The constant pressure chamber (6) is illuminated by a source of infrared light (11). Before passing through the chamber (6), the light source is attenuated by a filter or filters (12) which closely match the infrared absorption fingerprint of the gas or gases of interest. After passing through the chamber (6), via a window or windows (13), the light enters an infrared detector or detectors (14). Changes in the concentration of the gas or gases of interest cause corresponding changes in the level of infra-red light absorbed and thus a change in the level of electrical signal from the detector or detectors (14) which is related to the gas concentration.

For example, such an optical absorption gas sensor could be used to measure the concentration of carbon monoxide, carbon dioxide, and/or water vapour. With the use of multiple filters and multiple detectors, this optical absorption gas sensor can measure a plurality of different gas species simultaneously. Given as stated above it is possible to have multiple flow sensitive sensors (9) prior to the absorption sensor in the constant pressure chamber (6), it is possible to simultaneously measure most of the gasses emitted by internal combustion engines using a single sample line (2).

For the application here, the exact fraction of the flow blocked by the turbulator is not critical, since even for the lowest sample pressures encountered, the flow into capillary tube (8) is always many times smaller than the sample flow. There is however, an advantage to having the turbulator blockage area as small as possible, as the loss of frequency response of the instrument is then minimised.

A prior invention [GB1900645.1] improves upon [US5073753] by introducing a diffusive element (15) at the end of the sample tube continuation (5). This recovers pressure lost by wall friction in element (5) thus reducing the overall sample pressure sensitivity of the gas analyser in the mid and upper range of sample pressures. That invention may be combined with the present invention to reduce pressure sensitivity across the whole pressure range. An embodiment which combines these two features of turbulator (10) and diffuser (15) is shown in Figure 9.