One way to solve this problem might be to break it into injection gas and fluid sub-circuits. Then add a logic to represent their interaction. Perhaps something like the following diagram:

model GasLiquidMixture
  inner Modelica.Fluid.System system(m_flow_start = 1) annotation(Placement(visible = true, transformation(origin = {170, -50}, extent = {{-10, -10}, {10, 10}}, rotation = 0)));
  Modelica.Fluid.Pipes.StaticPipe pipe1(length = 20, diameter = 0.25, redeclare replaceable package Medium = Modelica.Media.IdealGases.SingleGases.Ar) annotation(Placement(visible = true, transformation(origin = {-90, -60}, extent = {{-10, -10}, {10, 10}}, rotation = 0)));
  Modelica.Fluid.Vessels.ClosedVolume mixingVolume(V = 1000, use_portsData = false, nPorts = 3, redeclare replaceable package Medium = Modelica.Media.IdealGases.SingleGases.Ar, p_start = 100000) annotation(Placement(visible = true, transformation(origin = {-10, -30}, extent = {{-10, -10}, {10, 10}}, rotation = 0)));
  Modelica.Fluid.Valves.ValveIncompressible valve(redeclare replaceable package Medium = Modelica.Media.IdealGases.SingleGases.Ar, dp_nominal = 10000, m_flow_nominal = 10) annotation(Placement(visible = true, transformation(origin = {-50, -60}, extent = {{-10, -10}, {10, 10}}, rotation = 0)));
  Modelica.Fluid.Pipes.StaticPipe pipe2(length = 20, diameter = 0.25, redeclare replaceable package Medium = Modelica.Media.Examples.TwoPhaseWater) annotation(Placement(visible = true, transformation(origin = {120, 30}, extent = {{-10, -10}, {10, 10}}, rotation = 0)));
  Modelica.Fluid.Vessels.ClosedVolume liquidSink(nPorts = 1, use_portsData = false, V = 1000, redeclare replaceable package Medium = Modelica.Media.Examples.TwoPhaseWater) annotation(Placement(visible = true, transformation(origin = {170, 50}, extent = {{-10, -10}, {10, 10}}, rotation = 0)));
  Modelica.Blocks.Sources.Ramp ramp1(offset = 0.01, height = 0.95, startTime = 2, duration = 5) annotation(Placement(visible = true, transformation(origin = {-90, -30}, extent = {{-10, -10}, {10, 10}}, rotation = 0)));
  Modelica.Fluid.Sources.Boundary_pT liquidSource(use_p_in = true, nPorts = 1, redeclare replaceable package Medium = Modelica.Media.Examples.TwoPhaseWater) annotation(Placement(visible = true, transformation(origin = {70, 30}, extent = {{-10, -10}, {10, 10}}, rotation = 0)));
  Modelica.Fluid.Sensors.Pressure gasPressure(redeclare replaceable package Medium = Modelica.Media.IdealGases.SingleGases.Ar) annotation(Placement(visible = true, transformation(origin = {20, 40}, extent = {{-10, -10}, {10, 10}}, rotation = 0)));
  Modelica.Fluid.Vessels.ClosedVolume gasSink(nPorts = 1, use_portsData = false, V = 1000, redeclare replaceable package Medium = Modelica.Media.IdealGases.SingleGases.Ar) annotation(Placement(visible = true, transformation(origin = {50, -30}, extent = {{-10, -10}, {10, 10}}, rotation = 0)));
  Modelica.Fluid.Sources.Boundary_pT compressedGasSource(redeclare replaceable package Medium = Modelica.Media.IdealGases.SingleGases.Ar, nPorts = 1, p = 500000, T = 298.15) annotation(Placement(visible = true, transformation(origin = {-130, -60}, extent = {{-10, -10}, {10, 10}}, rotation = 0)));
equation
  connect(compressedGasSource.ports[1], pipe1.port_a) annotation(Line(visible = true, origin = {-110, -60}, points = {{-10, 0}, {10, 0}}, color = {0, 127, 255}));
  connect(pipe1.port_b, valve.port_a) annotation(Line(visible = true, origin = {-70, -60}, points = {{-10, 0}, {10, 0}}, color = {0, 127, 255}));
  connect(ramp1.y, valve.opening) annotation(Line(visible = true, origin = {-59.667, -37.333}, points = {{-19.333, 7.333}, {9.667, 7.333}, {9.667, -14.667}}, color = {1, 37, 163}));
  connect(valve.port_b, mixingVolume.ports[1]) annotation(Line(visible = true, origin = {-20, -53.333}, points = {{-20, -6.667}, {10, -6.667}, {10, 13.333}}, color = {0, 127, 255}));
  connect(mixingVolume.ports[3], gasSink.ports[1]) annotation(Line(visible = true, origin = {80, -45}, points = {{-90, 5}, {-90, -30}, {-30, -30}, {-30, 5}}, color = {0, 127, 255}));
  connect(mixingVolume.ports[2], gasPressure.port) annotation(Line(visible = true, origin = {5, -25}, points = {{-15, -15}, {-15, -20}, {15, -20}, {15, 55}}, color = {0, 127, 255}));
  connect(gasPressure.p, liquidSource.p_in) annotation(Line(visible = true, origin = {40.25, 39}, points = {{-9.25, 1}, {-4.25, 1}, {-4.25, -1}, {17.75, -1}}, color = {1, 37, 163}));
  connect(liquidSource.ports[1], pipe2.port_a) annotation(Line(visible = true, origin = {95, 30}, points = {{-15, 0}, {15, -0}}, color = {0, 127, 255}));
  connect(pipe2.port_b, liquidSink.ports[1]) annotation(Line(visible = true, origin = {156.667, 33.333}, points = {{-26.667, -3.333}, {13.333, -3.333}, {13.333, 6.667}}, color = {0, 127, 255}));
  annotation(Diagram(coordinateSystem(extent = {{-150, -90}, {190, 75}}, preserveAspectRatio = true, initialScale = 0.1, grid = {5, 5}), graphics = {Text(visible = true, origin = {98.324, 59.557}, extent = {{-23.324, -4.557}, {23.324, 4.557}}, textString = "Fluid"), Text(visible = true, origin = {-90, 0}, extent = {{-23.324, -4.557}, {23.324, 4.557}}, textString = "Injection gas")}), Icon(coordinateSystem(extent = {{-100, -100}, {100, 100}}, preserveAspectRatio = true, initialScale = 0.1, grid = {10, 10}), graphics = {Rectangle(visible = true, lineColor = {0, 114, 195}, fillColor = {255, 255, 255}, extent = {{-100, -100}, {100, 100}}, radius = 25), Text(visible = true, textColor = {64, 64, 64}, extent = {{-150, 110}, {150, 150}}, textString = "%name")}));
end GasLiquidMixture;